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The greater omentum is derived from which of the following embryonic structures?
A. Dorsal mesoduodenum
B. Dorsal mesogastrium
C. Pericardioperitoneal canal
D. Pleuropericardial membranes
E. Ventral mesentery
Explanation:
The correct answer is B. Both the omental bursa and the greater omentum are derived from the dorsal
mesogastrium, which is the mesentery of the stomach region.
The dorsal mesoduodenum (choice A) is the mesentery of the developing duodenum, which later
disappears so that the duodenum and pancreas come to lie retroperitoneally.
The pericardioperitoneal canal (choice C) embryologically connects the thoracic and peritoneal canals.
The pleuropericardial membranes (choice D) become the pericardium and contribute to the diaphragm.
The ventral mesentery (choice E) forms the falciform ligament, ligamentum teres, and lesser omentum.
A PSA level is drawn from a 54-year-old man as part of a routine health evaluation. Which of the
following embryonic structures gives rise to the organ being screened for carcinoma?
A. Genital tubercle
B. Processus vaginalis
C. Testis cords
D. Tunica albuginea
E. Urogenital sinus
Explanation:
The correct answer is E. It is recommended that PSA levels (prostate-specific antigen) be
measured annually in men over age 50 to screen for prostatic carcinoma (and to record a
baseline level). The prostate is immediately derived from the prostatic urethra, which is
derived from the urogenital sinus.
The genital tubercle (choice A) gives rise to the glans penis.
The processus vaginalis (choice B) is a coelomic extension into the scrotal swelling that
carries with it extensions of the body wall to form the inguinal canal during the descent of the testes.
The testis cords (choice C) are composed of primitive germ cells, which give rise to
spermatogonia, and sex cord cells, which differentiate into Sertoli cells.
The tunica albuginea (choice D) is derived from mesenchyme and condenses to form the fibrous
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connective tissue capsule of the testis.
A newborn baby has projectile vomiting shortly after each feeding. It is determined that there is
obstruction of the digestive tract as a result of an annular pancreas. Annular pancreas is a
result of an abnormality in which of the following processes?
A. Rotation of the dorsal pancreatic bud around the first part of the duodenum
B. Rotation of the dorsal pancreatic bud around the second part of the duodenum
C. Rotation of the dorsal pancreatic bud around the third part of the duodenum
D. Rotation of the ventral pancreatic bud around the first part of the duodenum
E. Rotation of the ventral pancreatic bud around the second part of the duodenum
Explanation:
The correct answer is E. The ventral pancreatic bud normally rotates around the duodenum to
fuse with the dorsal pancreatic bud. Both pancreatic buds form from evaginations from the
second part of the duodenum, hence the rotation is around the second part of the duodenum. The
normal rotation is around the right side of the embryonic duodenum. Annular pancreas results
from the ventral pancreatic bud dividing and rotating around both the right and left sides of
the second part of the duodenum, thus encircling it.
The dorsal pancreatic bud (choices A, B, and C) does not rotate around the duodenum and
therefore is not the cause of annular pancreas.
The ventral pancreatic bud does not form from the first part of the duodenum (choice D) and
therefore does not rotate around this part of the duodenum.
A newborn infant has some of its abdominal viscera protruding through a defect in the abdominal wall.
Which of the following is the likely cause of this defect?
A. Failure of the intestinal loop to retract from the umbilical cord
B. Failure of the yolk stalk to degenerate
C. Failure of peritoneal fusion
D. Incomplete fusion of the lateral body folds
E. Umbilical herniation
Explanation:
The correct answer is D. During the fourth week of development, the lateral body folds move
ventrally and fuse in the midline to form the anterior body wall. Incomplete fusion results in
a defect that allows abdominal viscera to protrude from the abdominal cavity, a condition known
as gastroschisis.
During development, the midgut normally herniates into the umbilical cord and then subsequently
retracts into the abdominal cavity. Failure of the intestinal loop to retract from the
umbilical cord (choice A) results in omphalocele.
Failure of the yolk stalk to degenerate (choice B) results in an ileal (Meckel's) diverticulum
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or a vitelline fistula or cyst. In the early embryo, the gut tube is connected to the yolk sac
by a narrow connection known as the yolk stalk. Normally, this connection degenerates.
During development, certain peritoneal organs fuse with the posterior abdominal wall to become
secondarily retroperitoneal. Failure of this peritoneal fusion (choice C) will result in
certain organs that are normally immobile being mobile (e.g., mobile cecum).
Umbilical herniation (choice E) results from abdominal viscera protruding through a weakness in
the abdominal wall after development. Such protrusions are covered by subcutaneous fascia and
skin, distinguishing them from gastroschisis.
Autopsy of a baby who died minutes after birth demonstrates intestines within the thoracic cavity. Which
of the following most probably accounts for these findings?
A. Heterotopia
B. Hypertrophic pyloric stenosis
C. Large sliding hernia
D. Maldevelopment of the lungs
E. Partial absence of the diaphragm
Explanation:
The correct answer is E. In some infants, there is a weakness or partial-to-total absence of
the diaphragm (often on the left) which, if sufficiently large, can permit herniation of
abdominal contents into the thorax. Small defects are usually surgically correctable, but large
ones may cause rapidly lethal respiratory embarrassment in the newborn.
Heterotopia (choice A) is the presence of small amounts of normal tissue in abnormal sites.
Congenital hypertrophic pyloric stenosis (choice B) can cause persistent vomiting in infancy,
but does not cause herniation of intestines into the thorax.
Death in infancy due to intestinal herniation into the thorax is not related to sliding hernia
(choice C).
Maldevelopment of the lungs (choice D) is a result rather than the cause of this baby's problems.
During development, the formation of the kidney is induced by which of the following structures?
A. Allantois
B. Mesonephric duct
C. Mesonephros
D. Metanephric duct
E. Urogenital ridge
Explanation:
The correct answer is D. The metanephric duct (also known as the ureteric bud) is a
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diverticulum of the mesonephric duct. It grows to the metanephric mass of the urogenital ridge.
It induces the development of the metanephros, which will give rise to the excretory units of
the definitive kidney. The metanephros, in turn, induces the metanephric duct to divide into
the calyces and the collecting tubules.
The allantois (choice A) is an endodermal diverticulum of the yolk sac. It becomes incorporated
into the urogenital sinus and contributes to the formation of the urinary bladder. It is not
involved with the formation of the kidney.
The mesonephric duct (choice B) is a mesodermal duct into which the mesonephric tubules drain. The
mesonephric duct is the same as the pronephric duct, but at a later stage of development. The
mesonephric duct develops into the ductus deferens in the male. The metanephric duct is a
diverticulum of the mesonephric duct.
The mesonephros (choice C) is the embryonic kidney, which functions in the embryo before the
metanephros (or definitive kidney) develops. The mesonephros develops from the urogenital ridge.
The urogenital ridge (choice E) is a longitudinal elevation of the intermediate mesoderm. The
embryonic and adult kidneys, as well as the gonads, develop from this mesoderm.
Which of the following developmental abnormalities might account for unexplained small intestinal
bleeding?
A. Central nervous system heterotopia
B. Gastric heterotopia
C. Pancreatic heterotopia
D. Parathyroid heterotopia
E. Thyroid heterotopia
Explanation:
The correct answer is B. Heterotopic rests are small areas of normal tissue in abnormal sites.
These are usually clinically insignificant unless they form a noticeable mass or are
misdiagnosed in a biopsy (raising suspicion of metastatic cancer). One exception is a gastric
heterotopia, which typically occurs in the small intestine, and can produce enough acid to
cause a peptic ulcer in adjacent mucosa. The ulcer may be a source of gastrointestinal
bleeding.
Heterotopias of the CNS (choice A), parathyroid (choice D), and thyroid(choice E) can occur,
but would not be expected in the small intestine and would not be likely to cause bleeding if present.
Pancreatic heterotopia (choice C) can occur in the small intestine, but does not cause bleeding.
When examining a histological section of a normal ovary, a technician notices an oocyte surrounded by
several layers of follicular cells. A small antrum is present. Which of the following is the
correct term for the entire structure, composed of the oocyte, follicular cells, and antrum?
A. Corpus luteum
B. Graafian follicle
C. Primary follicle
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D. Primordial follicle
E. Secondary follicle
Explanation:
The correct answer is E. Follicles in different stages of maturation have different
appearances. The most primitive follicles, primordial follicles (choice D), are inactive
reserve follicles that contain primary oocytes (arrested in prophase of first meiotic division)
surrounded by a single layer of flattened follicular cells. Primary follicles (choice C), the
next stage, are slightly larger and contain a central oocyte surrounded by one or several
cuboidal follicular cells. When several small spaces in the follicular mass fuse to form the
antrum (follicular cavity), the follicle is termed a secondary follicle (choice E). The
secondary follicles continue to enlarge, and develop a more complex structure that includes
cumulus oophorus, corona radiata, theca interna, theca externa, and zona pellucida. The
Graafian follicle (choice B) is the mature form of the follicle, which extends through the
entire cortex and bulges out at the ovarian surface. After it ruptures and releases the ovum,
the corpus luteum (choice A)develops as the cells of the follicle and the theca interna cells
enlarge, become epithelioid, and secrete estrogen. The granulosa lutein cells contain yellow
pigment and secrete progesterone. If pregnancy does not occur, the corpus luteum eventually
degenerates; if pregnancy occurs, it is maintained throughout the pregnancy.
Which of the following cell types is derived from neuroepithelial cells?
A. Astrocytes
B. Enterochromaffin cells
C. Melanocytes
D. Odontoblasts
E. Schwann cells
Explanation:
The correct answer is A. Astrocytes and oligodendrocytes are both derived from glioblasts,
which, in turn, are derived from neuroepithelial cells. Other neuroepithelial cell derivatives
include neuroblasts and ependymal cells.
All the other choices are derived from neural crest cells. Other neural crest derivatives
include the neurons of the parasympathetic and sympathetic ganglia (including the adrenal
medulla), the dorsal root ganglia of the peripheral nervous system, the sensory ganglia of
cranial nerves V, VII, IX, and X, and the leptomeninges (pia and arachnoid).
Which of the following embryonic structures gives rise to the adrenal cortex?
A. Ectoderm
B. Endoderm
C. Mesoderm
D. Mesonephros
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E. Neural crest cells
Explanation:
The correct answer is C. The mesoderm gives rise to the adrenal cortex. In addition, it also
gives rise to connective tissue, cartilage, bone, muscle, blood and lymph vessels, kidneys, gonads,
serous membranes lining body cavities, and the spleen.
The ectoderm (choice A) gives rise to the central nervous system, peripheral nervous system,
epidermis and its appendages, mammary glands, pituitary gland, tooth enamel, and the neural crest.
The endoderm (choice B) gives rise to the parenchyma of the tonsils, thyroid and parathyroid
glands, thymus, liver, pancreas, the epithelial lining of the GI and respiratory tracts,
urinary bladder, urethra, and auditory tube.
The mesonephros (choice D) functions as an interim kidney in the embryo.
The neural crest cells (choice E) give rise to cells of the spinal and cranial nerves,
autonomic ganglia, melanocytes, leptomeninges, connective tissue and bone of branchial arch
origin, and the adrenal medulla.
A neuroscientist is investigating the development of the nervous system. In his experiments, he
microinjects a dye into the embryo of an animal subject in vivo. After birth, he performs
histological studies to determine the destination of the dye. In one animal subject, he locates
the dye in the dorsal horn of the spinal cord. Which of the following locations in the embryo
was the most likely site of the injection?
A. Alar plate
B. Basal plate
C. Neural crest
D. Rostral end of neural tube
E. Sulcus limitans
Explanation:
The correct answer is A. The spinal cord arises from the caudal end of the neural tube. During
development, an alar and a basal plate is formed, separated by a longitudinal groove called the
sulcus limitans (choice E). The alar plate forms the dorsal (posterior) part of the spinal cord
and becomes the sensory or afferent portion of the cord. The basal plate (choice B) is the
ventral (anterior) part of the cord, and becomes the motor, or efferent, portion of the spinal
cord, and therefore would contain anterior horn cells.
The neural crest (choice C) develops into multipolar ganglion cells of autonomic ganglia,
pseudounipolar cells of spinal and cranial nerve ganglia, leptomeningeal cells, Schwann cells,
melanocytes, chromaffin cells of the adrenal medulla, and odontoblasts.
The brain forms from the rostral end of the neural tube (choice D).
A newborn boy does not pass meconium until 48 hours after his birth. Two weeks later his mother reports
that he has not been passing stool regularly. Anorectal manometry reveals increased internal
anal sphincter pressure on rectal distention with a balloon. Radiographic studies reveal
massive dilation of the colon proximal to the rectum. This indicates a developmental
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abnormality in which of the following embryonic tissues?
A. Ectoderm
B. Endoderm
C. Neural crest
D. Neural ectoderm
E. Splanchnic mesoderm
Explanation:
The correct answer is C. The baby has Hirschsprung's disease, which is due to an absence of
ganglion cells in the wall of the colon. Neural crest cells contribute to the formation of many
adult structures. Among these are all of the postganglionic neurons of the autonomic nervous
system and the sensory neurons of the peripheral nervous system.
Ectoderm (choice A) forms the epidermis of the skin and the parenchymal cells of glands
associated with the skin such as the sweat glands, sebaceous glands, and mammary glands.
Endoderm (choice B) forms the epithelial lining of the gut tube and the parenchymal cells of
glands associated with the gut tube, such as the liver and pancreas.
Neural ectoderm (choice D) forms the central nervous system, the somatic motor neurons of the
peripheral nervous system, and the preganglionic neurons of the autonomic nervous system.
Splanchnic mesoderm (choice E) forms the visceral peritoneum, visceral pleura, visceral
pericardium, and the stroma and muscle of the wall of the gut, among other structures.
Physical examination of a young boy reveals discharge of urine from the umbilicus. The physician
concludes that the urachus has failed to fuse. Which of the following structures is the normal
adult remnant of the fused urachus?
A. Lateral umbilical fold
B. Medial umbilical fold
C. Medial umbilical ligament
D. Median umbilical fold
E. Median umbilical ligament
Explanation:
The correct answer is E. The urachus is the derivative of the allantoic duct, which passes
from the urogenital sinus to the umbilical cord. Normally, this duct fuses and is no longer
patent. The adult derivative is the median umbilical ligament, which lies in the midline along
the interior surface of the anterior abdominal wall. It passes from the upper end of the
bladder to the umbilicus.
The lateral umbilical fold (choice A) is the fold of parietal peritoneum that covers the
inferior epigastric artery and vein on the interior surface of the anterior abdominal wall.
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The medial umbilical fold (choice B) is the fold of parietal peritoneum that covers the medial
umbilical ligament, the adult derivative of the umbilical artery (see below).
The medial umbilical ligament (choice C) is the adult derivative of the distal portion of the
umbilical artery. The umbilical artery arises from the internal iliac artery. It passes along
the bladder and then the anterior abdominal wall to reach the umbilicus. Prenatally, this
artery carries fetal blood to the placenta, where it gains oxygen and nutrients. Postnatally,
the proximal part of the umbilical artery remains patent and supplies blood to the superior
surface of the bladder. Distal to the bladder, the artery becomes fibrotic and is known as the
medial umbilical ligament.
The median umbilical fold (choice D) is the fold of parietal peritoneum that covers the median
umbilical ligament, the adult derivative of the urachus.
A male infant presents with flattened facial features, low set ears, and deformities of the feet. The
lungs are underdeveloped. The pregnancy was complicated by severe oligohydramnios. The most likely cause
of this condition is a malformation of the
A. paramesonephric ducts
B. pronephros
C. ureteric buds
D. urogenital sinus
E. urorectal septum
Explanation:
The correct answer is C. This infant suffers from Potter syndrome, caused by an absence of both
kidneys. This is often caused by a failure of the ureteric buds to develop. The ureteric buds
form the ureters, renal pelvis, calyces, and collecting tubules. The collecting tubules induce
the formation of metanephric vesicles, which differentiate into the tubular components of the
nephron.
The paramesonephric ducts (choice A) form the uterine tubes, uterus, and the upper third of the
vagina.
The pronephros (choice B) is a transient collection of cells that disappears during the fourth week of
development. It does not form the permanent kidney.
The urogenital sinus (choice D) forms the urinary bladder (in both sexes). It also forms the
prostatic urethra, prostate, membranous urethra, bulbourethral glands, penile urethra (proximal
part) and Littre's glands in the male. In the female it forms most of the urethra, urethral
glands, paraurethral glands, vestibule of the vagina and greater vestibular glands.
The urorectal septum (choice E) partitions the cloaca into the urogenital sinus and rectum (and
upper anal canal).
A 47-year-old man presents to his neurologist with an unsteady broad-based gait and slow, slurred
speech. Neurological examination reveals dysdiadochokinesis, intention tremor, hypotonia, and
nystagmus. The patient's lesion is in a brain structure that derives from which of the following
embryonic structures?
A. Diencephalon
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B. Mesencephalon
C. Metencephalon
D. Myelencephalon
E. Telencephalon
Explanation:
The correct answer is C. The patient in question has a cerebellar lesion. Cerebellar
dysfunction can lead to a variety of motor dysfunctions, including truncal ataxia (appearing
similar to the gait of an intoxicated individual), intention tremor (uncontrolled shaking of
affected extremity present only with purposeful movement), dysdiadochokinesia (the inability to
perform rapid and regular alternating movements), dysmetria (inability to stop movements at the
desired point), dysarthria (ataxic speech), hypotonia, and nystagmus.
During the fourth week of embryonic development, the anterior end of the neural tube develops
three vesicles, the prosencephalon (forebrain), the mesencephalon (midbrain), and the
rhombencephalon (hindbrain). By the sixth week, five vesicles (listed in the answer options)
have developed. The rhombencephalon has now developed into the metencephalon and
myelencephalon. The cerebellum and pons derive from the metencephalon.
The diencephalon (choice A), which is derived from the prosencephalon, develops into the thalamus,
hypothalamus, epithalamus, subthalamus (everything with the word "thalamus"), posterior lobe of
the pituitary, and neural retina.
The mesencephalon (choice B), or midbrain, is the only brain vesicle that does not produce a secondary
vesicle; it remains the mesencephalon.
The myelencephalon (choice D), which is derived from the rhombencephalon, develops into the medulla
oblongata.
The telencephalon (choice E), which is derived from the prosencephalon, develops into the cerebral
hemispheres (cerebral cortex, basal ganglia, and deep white matter).
Evaluation of an infant with a variety of congenital abnormalities reveals hypocalcemia due to a lack of
parathyroid hormone. On x-ray, the thymic shadow is absent. A failure of development and
differentiation of which of the following embryonic structures would most likely be responsible
for the observed presentation?
A. Second pharyngeal arch
B. Second pharyngeal cleft
C. Second pharyngeal pouch
D. Third pharyngeal arch
E. Third pharyngeal pouch
Explanation:
The correct answer is E. The third pharyngeal pouch normally gives rise to the inferior
parathyroid glands and the thymus. Cells from these pouches migrate caudally in the embryo to
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the eventual location of these organs. The superior parathyroid glands are derived from the
fourth pharyngeal pouch. The absence of these organs is found in DiGeorge syndrome, which
typically presents with immunodeficiency and hypoparathyroidism.
The second pharyngeal arch (choice A) gives rise to several skeletal and muscular structures.
These include the stapes, styloid process, stylohyoid ligament, a portion of the hyoid bone,
the stapedius muscle, stylohyoid muscle, posterior digastric muscle, and all of the muscles of
facial expression.
The second pharyngeal cleft (choice B) is normally obliterated. A persistence of this cleft may
result in a lateral cervical cyst, sinus, or fistula.
The second pharyngeal pouch (choice C) gives rise to the tonsillar fossa and the palatine tonsils.
The third pharyngeal arch (choice D) gives rise to most of the hyoid bone and the
stylopharyngeus muscle. The third aortic arch, which passes through the third pharyngeal arch,
gives rise to the common carotid artery and the internal carotid artery.
Most of the oocytes in the ovary of a prepubescent girl are in which meiotic stage?
A. Anaphase of the second meiotic division
B. Metaphase of the first meiotic division
C. Metaphase of the second meiotic division
D. Prophase of the first meiotic division
E. Telophase of the first meiotic division
Explanation:
The correct answer is D. The first meiotic division is the "reduction" meiotic division, in
which the diploid complement of DNA is reduced to a haploid complement. The bulk of oocytes in
premenopausal women, girls, and babies are arrested at prophase of the first meiotic division.
Postmenopausal women have very few viable oocytes.
The heart of an embryo first begins beating at which of the following ages?
A. 2 weeks
B. 3 weeks
C. 4 weeks
D. 6 weeks
E. 8 weeks
Explanation:
The correct answer is C. While the third-week embryo is a primitive trilaminar plate, in the
fourth week, the heart begins to form and begins beating almost immediately. Hematopoiesis
occurs in the yolk sac, and a primitive circulatory system connects the capillary plexuses of
the yolk sac and chorion to the embryo. Partitioning of the atrium also begins in the fourth
week. During the fifth week, cardiac septa form and the atrioventricular (AV) cushions fuse. By
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the sixth week, the heart is close to fully formed. This early sequence for the heart explains
why it is so difficult to try to prevent congenital malformations of the cardiovascular system
from occurring, since the mother of a 6-week-old fetus is only about 8 weeks from her last
menstrual period, and may have assumed that she just "missed a period" (a very common
phenomenon) for reasons other than pregnancy.
A 17-year-old male is examined by a physician, who notes a mass at the back of the young man's tongue.
The physician biopsies the mass, and the pathology report comes back with a diagnosis of normal
thyroid tissue. The occasional presence of such tissue at the back of the tongue is related to
the embryonic origin of the thyroid near which of the following structures?
A. First pharyngeal pouch
B. Foramen cecum
C. Nasolacrimal duct
D. Second pharyngeal arch
E. Third pharyngeal pouch
Explanation:
The correct answer is B. The thyroid gland originates as a mass of endodermal tissue near the
foramen cecum, which is near the tuberculum impar (which becomes the central part of the
tongue). During development, the thyroid descends in front of the pharynx, maintaining a
connection to the tongue via the thyroglossal duct. Usually, the thyroglossal duct disappears.
Uncommonly, residual ectopic thyroid tissue can be left anywhere along the path, including at
the back of the tongue. (In rare patients, all of the thyroid tissue remains at this site,
forming a mass that should not be excised, for obvious reasons!)
The first pharyngeal pouch (choice A) develops into the middle ear and eustachian tube.
The nasolacrimal ducts (choice C) connect the eyes to the mouth.
The second pharyngeal arch (choice D) develops into many muscles of the face and styloid
process of the temporal bone.
The third pharyngeal pouch (choice E) develops into the thymus and inferior glands.
A 2-year-old-boy is brought to the pediatrician by his mother because he has had several episodes of
rectal bleeding. Evaluation with a technetium-99m perfusion scan reveals a 3-cm ileal
outpouching located 60 cm from the ileocecal valve. This structure likely contains which of the
following types of ectopic tissue?
A. Duodenal
B. Esophageal
C. Gastric
D. Hepatic
E. Jejunal
Explanation:
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The correct answer is C. This little boy has a Meckel's diverticulum, an ileal outpocketing
typically located within 50-75 cm of the ileocecal valve. It is a congenital anomaly resulting
from the persistence of the vitelline (omphalomesenteric) duct. Approximately half cause
ulceration, inflammation, and gastrointestinal bleeding due to the presence of ectopic acidsecreting gastric epithelium. Pancreatic tissue may sometimes occur in these diverticula as
well. Note that this is the most common type of congenital gastrointestinal anomaly.
Something else to keep in mind: A favorite question attendings ask on the wards is the rule of
2's associated with Meckel's diverticulum: it occurs in about 2% of children, occurs within
approximately 2 feet of the ileocecal valve, contains 2 types of ectopic mucosa (gastric and
pancreatic), and its symptoms usually occur by age 2.
All of the other answer choices have no relationship to Meckel's diverticulum.
During embryological development, hematopoiesis occurs in different organs at different times. Which of
the following are the correct organs, in the correct sequence, at which hematopoiesis occurs
embryologically?
A. Amnion, yolk sac, placenta, bone marrow
B. Placenta, liver and spleen, yolk sac, bone marrow
C. Placenta, spleen and lymphatic organs, bone marrow
D. Yolk sac, bone marrow, liver and spleen
E. Yolk sac, liver, spleen and lymphatic organs, bone marrow
Explanation:
The correct answer is E. By the third week of development, hematopoiesis begins in the blood
islands of the yolk sac. Beginning at 1 month of age and continuing until 7 months of age,
blood elements are also formed in the liver. Hematopoiesis occurs in the spleen and lymphatic
organs between 2 and 4 months, and in the bone marrow after 4 months.
A neuroscientist discovers a way to selectively label neural crest cells in a developing laboratory
animal. After birth, he sacrifices the animal and examines the tissue to search for labeled
cells. Which of the following cell types will contain the label?
A. Astrocytes
B. Ependymal cells
C. Microglia
D. Oligodendroglia
E. Pseudounipolar cells
Explanation:
The correct answer is E. Pseudounipolar cells of spinal and cranial nerve ganglia derive from
the neural crest. Other cell types that derive from neural crest include Schwann cells,
multipolar ganglion cells of autonomic ganglia, chromaffin cells of the adrenal medulla,
odontoblasts, melanocytes, and cells that make the leptomeninges (pia and arachnoid). All of
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the other cell types listed derive from the neural tube.
Physical examination of a 6-year-old child reveals a heart murmur. An echocardiogram shows a ostium
primum type of atrial septal defect. This defect results from failure of the
A. ostium primum to form within the septum primum
B. ostium secundum to form within the septum primum
C. septum primum to fuse with the endocardial cushions
D. septum primum to fuse with the septum secundum
E. septum secundum to fuse with the endocardial cushions
Explanation:
The correct answer is C. The septum primum (first interatrial septum) develops by growing from
the cranial end of the embryonic atrium toward the endocardial cushions. The gap that exists
between the two atria during this period is the ostium primum. As the septum primum continues
its growth, the ostium primum gets smaller. When the septum primum completes its growth and
completely fuses with the endocardial cushions, the ostium primum is closed. Failure of the
septum primum to fuse completely with the endocardial cushions leaves a persistent ostium
primum, known as a primum type atrial septal defect.
The ostium primum does not form within the septum primum (choice A). The ostium primum is the
communication between the two atria that exists during the formation of the septum primum. That
is, the ostium primum is the space within the developing atrium not yet occupied by the septum
primum.
The ostium secundum normally forms within the septum primum (choice B) before the ostium primum
closes by fusion of the septum primum with the endocardial cushions. Failure of the ostium
secundum to form would result in embryonic death, because there would be no pathway for blood
to pass from the right atrium to the left atrium when the ostium primum closes, thus depriving
the embryo of oxygenated blood.
Most of the septum primum normally disappears. The part that remains forms the valve of the
foramen ovale. This part of the septum primum normally does not fuse with the septum secundum
(choice D) during prenatal life. After birth, the valve of the foramen ovale is pushed against
the septum secundum as a result of the increased pressure in the left atrium. This achieves
functional closure of the foramen ovale. Fusion does not normally occur at this time, but
usually occurs later in life in most people. In some people, however, complete fusion never
occurs (probe patency).
The septum secundum normally does not fuse with the endocardial cushions (choice E).
The notochord forms on approximately what day after conception?
A. Day 2
B. Day 7
C. Day 17
D. Day 28
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E. Day 60
Explanation:
The correct answer is C. Formation of the notochord is a third week event, occurring as a
relatively early part of development of the trilaminar disc. More specifically, notochord
formation begins about day 16 and the neuroenteric canal forms on about day 18.
Two days after conception (choice A), implantation has not yet occurred.
By seven days after conception (choice B), the blastocyst has usually implanted in the endometrium.
By day 28 (choice D), gastrulation has occurred, and organogenesis is under way.
By day 60 (choice E), organogenesis is complete.
Which of the following placental components is derived from the mother rather than the fetus?
A. Chorion
B. Cytotrophoblast
C. Lacunar network
D. Primary villus
E. Syncytiotrophoblast
Explanation:
The correct answer is C. The placenta is derived from both maternal and fetal tissues. The
mother produces the capillary bed that forms the lacunar network of the placenta.
The fetus produces both the cytotrophoblast (choice B) and the syncytiotrophoblast (choice E),
which together form the chorion (choice A). The earliest projections of this are called the primary
villi (choice D).
A baby born to an alcoholic mother had microcephaly and cardiac abnormalities and died despite
resuscitative efforts. During which of the following periods of pregnancy is alcohol most teratogenic?
A. First 2 weeks
B. 3rd to 9th weeks
C. 9th to 12th weeks
D. 12th to 16th weeks
E. 16th to 20th weeks
Explanation:
The correct answer is B. Embryonic tissue is most susceptible to teratogens during the 3rd
through 9th weeks of pregnancy. This is when organogenesis occurs, and when most major
congenital anomalies occur. But since brain development occurs throughout pregnancy, it is
wisest for a pregnant woman to avoid alcohol for all 9 months of gestation.
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The first 2 weeks (choice A) of pregnancy are not generally associated with teratogenicity,
unless so many of the cells of the conceptus are irreversibly damaged that death results. Otherwise, the
abundance of undifferentiated cells present are able to compensate for the damage.
The 9th through 12th weeks (choice C) are associated with minor congenital anomalies, including
those of the eyes, palate, teeth, and ears.
The 12th through 16th weeks (choice D) and 16th through 20th weeks (choice E) are associated
with minor congenital anomalies, including those of the eyes, teeth, ears, and external
genitalia. Minor CNS anomalies may initiate during the 20th week.
A newborn child has an abnormally formed mandible, ears, and palate. He is diagnosed with
mandibulofacial dysostosis. This syndrome is due to abnormal development of which of the following
structures?
A. First pharyngeal arch
B. First pharyngeal cleft
C. Second pharyngeal arch
D. Second pharyngeal cleft
E. Third pharyngeal cleft
F. Third pharyngeal pouch
Explanation:
The correct answer is A. The pharyngeal arches are outpouchings of tissue visible on the
external neck of the embryo. They are separated by the pharyngeal clefts (each one caudal to
its arch). The pharyngeal pouches are the outpouchings of the pharynx visible inside the embryo
that correspond to the arches. The defect described is due to abnormal development of the
derivatives of the first pharyngeal arch. It is thought that the initial defect may be
insufficient migration of neural crest cells.
In the newborn nursery, a neonate develops a bluish color when crying. Which of the following congenital
defects may be the cause of the cyanosis?
A. Patent ductus arteriosus
B. Persistent truncus arteriosus
C. Primum type atrial septal defect
D. Secundum type atrial septal defect
E. Ventricular septal defect
Explanation:
The correct answer is B. A persistent truncus arteriosus results from a failure of the
aorticopulmonary septum to form. The aorticopulmonary septum divides the truncus arteriosus into the
ascending aorta and the pulmonary trunk. If this septum fails to form, the single truncus arteriosus
will receive blood from both the right and left ventricles, allowing the deoxygenated blood to be mixed
16
with the oxygenated blood. The systemic circulation therefore receives blood that is not fully
oxygenated, causing cyanosis.
A patent ductus arteriosus (choice A) results in blood passing from the aorta to the pulmonary trunk
postnatally. This left-to-right shunt does not cause cyanosis. Cyanosis is caused by right-toleft shunts. Prenatally, the ductus arteriosus allows the passage of blood from the pulmonary
trunk to the aorta. After birth, when the pressure gradient reverses and the pressure is higher
on the left side of the heart, the flow in a patent ductus arteriosus reverses and becomes a
left-to-right shunt.
Atrial septal defects (choices C and D) allow for left-to-right shunting of blood postnatally
because the pressure is higher in the left atrium than in the right atrium. Only right-to-left shunts
cause cyanosis.
Ventricular septal defects (choice E) also produce a left-to-right shunt because of the leftto-right pressure gradient. Therefore, these are noncyanotic defects.
Ultrasonographic examination of a 7-month-old fetus demonstrates limb and facial deformities. The amount
of amniotic fluid is estimated to be about 10 mL, considerably lower than normal. A failure of
development of which of the following organs can initiate the deformation sequence experienced
by this fetus?
A. Duodenum
B. Esophagus
C. Heart
D. Kidneys
E. Lungs
Explanation:
The correct answer is D. The sequence is Potter's syndrome, in which bilateral renal agenesis leads to
oligohydramnios (an abnormally low amount of amniotic fluid) because the kidneys do not produce normal
amounts of urine, which contributes to amniotic fluid. Oligohydramnios causes uterine compression of the
fetus, which in turn causes limb deformities, breech position, and pulmonary hypoplasia.
Duodenal (choice A) and esophageal (choice B) atresia can cause polyhydramnios, rather than
oligohydramnios, since the amniotic fluid cannot be swallowed, but continues to be produced by
the kidneys.
The heart (choice C) is not directly involved in Potter's syndrome, although major
malformations of the heart may accompany major malformations of the kidneys.
The lungs (choice E) become secondarily hypoplastic as a consequence of inadequate amniotic
fluid, rather than as a primary malformation.
A low-birth-weight neonate, who passed meconium on the first day of life, is started on formula at 2
days of age. The infant develops abdominal distension and tenderness, accompanied by findings suggestive
of sepis, including hypotension and neutrophilia of the blood. What is most likely to be seen at
emergency surgery?
A. A massive thickening of the pylorus
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B. A massively dilated colon
C. Bowel loops in the chest cavity
D. Gangrene of the terminal ileum and ascending colon
E. Normal-appearing gastrointestinal tract
Explanation:
The correct answer is D. The infant has necrotizing enterocolitis, which can affect infants any
time in the first few months of life, but is most common around the time an infant is started
on oral foods. The functional immaturity of the neonatal gut apparently leaves it particularly
vulnerable to a combination of ischemic injury and colonization by pathogenic organisms.
Formula-fed infants appear to be more vulnerable than breast-fed infants, possibly due to the
absence of maternal antibodies or other immunoprotective features in formula. Premature and
low-birth-weight infants are also more likely to develop this condition. Affected infants may
have either mild gastrointestinal illness or develop frank, life-threatening gangrene of the
terminal ileum and ascending colon.
Thickening of the pylorus (choice A) is a feature of congenital hypertrophic pyloric stenosis,
which typically presents with vomiting and regurgitation in an older infant.
A massively dilated colon (choice B) suggests Hirschsprung's disease, which may present in a
similar fashion to necrotizing enterocolitis. The two conditions can usually be distinguished
by noting that meconium is usually not passed prior to feeding of the neonate in Hirschsprung's
disease.
Bowel loops in the chest cavity (choice C) are a feature of congenital diaphragmatic hernia,
which causes respiratory distress and, often, death in neonates.
The gastrointestinal tract would be unlikely to appear normal (choice E) in a case such as this.
A newborn baby has a large, cystic "mass" of the lumbar region. The mass is found to consist of CSFfilled meningeal tissue that has herniated through a vertebral defect. The child moves all her
extremities and responds to external stimulation in a normal fashion. The term best used to
describe this child's lesion is which of the following?
A. Encephalocele
B. Meningocele
C. Meningomyelocele
D. Spina bifida aperta
E. Spina bifida occulta
Explanation:
The correct answer is B. Meningocele occurs due to a failure of closure of the caudal portion
of the neural tube. In contrast to the more serious meningomyelocele, in which the spinal cord
also herniates through the vertebral defect, meningocele is typically associated with only
minimal neurological damage and is relatively easy to treat surgically.
Encephalocele (choice A) implies herniation of the brain through a bony defect in the skull
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caused by defective closure of the cranial end of the neural tube.
Meningomyelocele (choice C) includes herniation of the spinal cord through a vertebral defect.
Associated neurological deficits are usually severe.
In spina bifida aperta (choice D), there is a complete failure of fusion of the neural plate
associated with malformation of the vertebral arches such that the underdeveloped neural plate
is covered only with skin. No mass lesion is present, and the patient typically has major
neurologic deficits.
Spina bifida occulta (choice E) is the mildest of the neural tube defects, typically consisting
of a small vertebral defect with a normal spinal cord and meninges. The defect is often
associated with a skin dimple that overlies adipose tissue, hair follicles, or a sinus.
From which of the following fetal vessels do the umbilical arteries arise?
A. Aorta
B. Carotid arteries
C. Ductus arteriosus
D. Iliac arteries
E. Pulmonary arteries
Explanation:
The correct answer is D. The paired umbilical arteries arise from the iliac arteries. They
supply unoxygenated fetal blood to the placenta. The single umbilical vein takes the newly
oxygenated fetal blood from the placenta to the liver and then to the inferior vena cava via
the ductus venosus.
A newborn baby has a prominent defect at the base of his spine through which his meninges and spinal
cord protrude. A failure of which of the following processes is the most common cause of this
type of defect?
A. Development of primary vertebral ossification centers
B. Development of the body
C. Development of the pedicle
D. Development of the superior articular process
E. Fusion of the vertebral arches
Explanation:
The correct answer is E. The condition described is spinal bifida with myelomeningocele. A
failure of the posteriorly located vertebral arches to fuse posteriorly causes spina bifida,
which can vary in severity from a completely asymptomatic minor abnormality to protrusion of
the spinal cord and roots through an open skin defect, with a very real risk of minor trauma or
infection causing paralysis.
Failure of development of one of the paired primary ossification centers (choice A) of the body
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can produce very severe scoliosis.
The bodies of the vertebrae (choice B) are the stacking ovoid structures on the anterior aspect
of the spinal canal.
The pedicles (choice C) attach the body ring that surrounds the spinal canal to the body of the
vertebrae.
The superior articular process (choice D) articulates with the inferior articular facet of the
vertebra above it.
An infant is born with an abnormally developed falciform ligament. The hepatogastric and hepatoduodenal
ligaments are also malformed. These developmental anomalies are most likely due to abnormal
development of the
A. dorsal mesoduodenum
B. dorsal mesogastrium
C. pericardioperitoneal canal
D. pleuropericardial membranes
E. ventral mesentery
Explanation:
The correct answer is E. The ventral mesentery forms the falciform ligament, ligamentum teres,
and lesser omentum, which can be divided into the hepatogastric and hepatoduodenal ligament.
The dorsal mesoduodenum (choice A) is the mesentery of the developing duodenum, which later
disappears so that the duodenum and pancreas lie retroperitoneally.
Both omental bursa and the greater omentum are derived from the dorsal mesogastrium (choice B),
which is the mesentery of the stomach region.
The pericardioperitoneal canal (choice C) embryologically connects the thoracic and peritoneal canals.
The pleuropericardial membranes (choice D) become the pericardium and contribute to the diaphragm.
A 7-year-old patient presents with a mass in the anterior midline of the neck, slightly above the
larynx. The mass is mobile and elevates upon protrusion of the tongue. This mass is most likely
a cyst that developed from which of the following embryonic structures?
A. First pharyngeal cleft
B. First pharyngeal pouch
C. Second pharyngeal cleft
D. Second pharyngeal pouch
E. Thyroglossal duct
Explanation:
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The correct answer is E. The thyroglossal duct develops as an evagination of the floor of the
pharynx in the region where the tongue develops. The adult foramen cecum of the tongue marks
the site of this evagination. The distal end of this duct normally forms the thyroid gland; the
proximal part of the duct normally degenerates. Failure of a part of the duct to degenerate may
lead to a thyroglossal duct cyst or median cervical cyst, as seen in this patient.
The first pharyngeal cleft (choice A) forms the external ear canal. This cleft normally remains
patent.
The first pharyngeal pouch (choice B) forms the middle ear cavity and the auditory tube. This
pouch normally remains patent.
The second pharyngeal cleft (choice C) normally does not remain patent. It is typically covered
over by the overgrowth of the second pharyngeal arch. If part of this pouch does remain patent,
it may form a lateral cervical cyst, which is seen on the lateral side of the neck along the
posterior border of the sternocleidomastoid muscle.
The second pharyngeal pouch (choice D) forms the tonsillar fossa of the pharynx. The pharyngeal
mucosa in this area arises from the endoderm of the pouch. Ingrowth of mesoderm cells results
in the formation of the palatine tonsil.
A researcher employs site-directed mutagenesis to produce mutations in a gene that is important for
development of the ureteric bud in a rat embryo. The embryo develops abnormally, and is
delivered stillborn twenty days later. Examination of the stillborn fetus would likely reveal
absence of which of the following renal structures?
A. Ascending loop of Henle
B. Bowman's capsule
C. Collecting duct
D. Descending loop of Henle
E. Glomerular tuft
Explanation:
The correct answer is C. The collecting system, including the collecting ducts, minor and
major calyces, and the renal pelvis are all derived from the ureteric bud. All of the other
structures listed are derived from metanephric mesoderm.
Autopsy of an infant who died with multiple congenital anomalies reveals a malformed heart. The aorta
arises from a morphologic right ventricle located on the right side of the heart. The pulmonary
artery overrides a ventricular septal defect. Which of the following terms best describes this
infant's heart?
A. "Common pattern" of transposition of the great arteries
B. "Corrected pattern" of transposition of the great arteries
C. Taussig-Bing malformation
D. Tetralogy of Fallot
E. Truncus arteriosus
21
Explanation:
The correct answer is C. The heart has a Taussig-Bing malformation, which is considered to be a
variant of transposition of the great arteries because the aorta arises from the morphologic right
ventricle.
In the "common pattern" of transposition (choice A), the aorta arises from a morphologic right
ventricle on the right side of the heart, and the pulmonary artery arises from a morphologic
left ventricle on the left side of the heart.
In the "corrected pattern" of transposition (choice B), the aorta arises from a morphologic
right ventricle on the left side of the heart, and the pulmonary artery arises from a
morphologic left ventricle on the right side of the heart.
In tetralogy of Fallot (choice D), the aorta overlies a septal defect, and the proximal
pulmonary artery is stenotic.
In truncus arteriosus (choice E), a single large vessel overlies a ventricular septal defect.
In a genotypic male, the testes fail to develop, and do not secrete testosterone or Müllerian regression
factor. Which of the following best describes the in utero reproductive system development of this
individual?
A. Both male- and female-type internal reproductive tracts and male-type external genitalia
B. Female-type internal reproductive tract and female-type external genitalia
C. Female-type internal reproductive tract and male-type external genitalia
D. Male-type internal reproductive tract and female-type external genitalia
E. Male-type internal reproductive tract and male-type external genitalia
Explanation:
The correct answer is B. The description is that of gonadal dysgenesis. In the absence of
testosterone, the Wolffian ducts will regress and fail to differentiate into normal male
internal reproductive tracts. In the absence of Müllerian regression factor, the Müllerian
ducts will automatically differentiate into oviducts and a uterus. Differentiation of the male
external genitals is dependent on adequate dihydrotestosterone (via an action of 5 αreductase on testosterone). In the absence of testosterone, female-type external genitalia will
develop.
Selective dysgenesis of the Sertoli cells could produce the situation described in choice A.
Normal Leydig cells would secrete testosterone and produce normal male-type internal and
external tracts. However, the absence of Müllerian regression factor, which is secreted by the
Sertoli cells, would allow formation of female-type internal structures as well.
Female-type internal reproductive tract and male-type external genitalia (choice C) would not
be likely to occur under any circumstances.
The situation described in choice D could occur with 5 α-reductase deficiency. Normal
male-type internal tracts can form because there is no requirement for dihydrotestosterone.
Müllerian regression factor will prevent differentiation of female-type internal tracts. Since
differentiation of the normal male external genitals requires dihydrotestosterone, 5 α-
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reductase deficiency will lead to feminization.
The situation described in choice E is normal, and would not occur in the individual described
who has testicular dysgenesis.
A mutation affecting the development of the diencephalon could interfere with the secretion of which of
the following hormones?
A. Adrenocorticotrophic hormone (ACTH)
B. Epinephrine
C. Oxytocin
D. Prolactin
E. Thyroid stimulating hormone (TSH)
Explanation:
The correct answer is C. The neurohypophysis (posterior pituitary) is derived from an evagination of
diencephalic neurectoderm. This structure is responsible for releasing oxytocin and vasopressin
to the general circulation. Both hormones are synthesized in cell bodies contained within the
hypothalamus.
ACTH (choice A), prolactin (choice D), and TSH (choice E) are all synthesized and released by
the anterior pituitary, or adenohypophysis, which is derived from an evagination of the
ectoderm of Rathke's pouch, a diverticulum of the primitive mouth. Remnants of this pouch may
give rise to a craniopharyngioma in later life.
Epinephrine (choice B) is synthesized and released into the circulation by the adrenal medulla,
a neural crest derivative.
Which of the following characteristics is similar for spermatogenesis and oogenesis?
A. Age at which meiosis begins
B. Amount of cytoplasm retained
C. DNA replication during meiosis
D. Length of prophase I
E. Transmission to fetus of mitochondrial DNA
Explanation:
The correct answer is C. The actual process of manipulation of DNA and chromosomes during
meiosis is very similar in spermatogenesis and oogenesis. However, the processes also differ in many
other respects:
In oogenesis, the process of meiosis begins before birth, and arrests between birth and puberty
in prophase I. In contrast, spermatogenesis does not begin until puberty (see choice A).
The egg retains a large volume of cytoplasm (choice B), while nearly all the cytoplasm is
stripped during formation of a sperm.
23
As noted above, in oogenesis, meiosis is arrested in prophase I, which is consequently very
prolonged in the female. In spermatogenesis, meiosis is completed in a much shorter time (choice D).
Both the egg and the sperm have mitochondria, but those of the sperm are left outside when the
sperm nucleus enters the egg, and consequently do not contribute to the mitochondrial genome of
the fetus. Instead, the mitochondria are transmitted from the egg to the fetus (choice E).
Traits coded for by mitochondrial DNA are therefore inherited in a matrilineal fashion.
Which of the following respiratory system components is derived from neural crest?
A. Endothelial cells
B. Epithelium of primary bronchi
C. Laryngeal cartilage
D. Tracheal glands
E. Type I pneumocytes
Explanation:
The correct answer is C. Laryngeal cartilages (e.g., the thyroid, cricoid, arytenoid
cartilages) are derived from neural crest.
The endothelial cells (choice A) in the simple squamous epithelium that lines the pulmonary
capillaries are derived from visceral mesoderm.
The epithelial lining of primary bronchi (choice B) is derived from endoderm.
Tracheal glands (choice D) and epithelium both derive from endoderm.
Type I pneumocytes (choice E) are derived from endoderm.
The smooth part of the right atrium derives from which of the following embryonic structures?
A. Bulbus cordis
B. Primitive atrium
C. Primitive ventricle
D. Sinus venosus
E. Truncus arteriosus
Explanation:
The correct answer is D. The smooth part of the right atrium (the sinus venarum) is derived
from the sinus venosus. The coronary sinus and the oblique vein of the left atrium also derive
from the sinus venosus.
The bulbus cordis (choice A) gives rise to the smooth part of the right ventricle (conus
arteriosus) and the smooth part of the left ventricle (aortic vestibule).
24
The primitive atrium (choice B) gives rise to the trabeculated part of the right and left atria.
The primitive ventricle (choice C) gives rise to the trabeculated part of the right and left
ventricles.
The truncus arteriosus (choice E) gives rise to the proximal part of the aorta and the
proximal part of the pulmonary artery.
A patient, who appears to be female, is found to be 46,XY. The patient's vagina is very shallow, ending
in a blind pouch, and there are palpable masses in the labia. The diagnosis of testicular
feminization syndrome is made. Which of the following was most likely present during the early
fetal life of this individual?
A. A streak ovary
B. A uterus
C. An oviduct
D. Depressed levels of testosterone
E. MIF (Mullerian inhibitory factor)
Explanation:
The correct answer is E. Testicular feminization is a disorder of the androgen receptor.
Phenotypically, the patient appears female, but has a blindly ending vagina and lacks a uterus or other
female internal reproductive organs. The patient has an XY genotype. Since the gene for testes
determining factor (TDF) is on the Y chromosome, TDF will cause the indifferent gonad to develop into a
testis containing Sertoli cells. Sertoli cells at this stage will secrete MIF, a substance that
suppresses the paramesonephric ducts, preventing the formation of female internal reproductive organs.
This patient would not have a streak ovary (choice A), a finding in Turner's syndrome that is
associated with a 45,XO genotype. In fact, the patient would have testes, since the genetic complement
contained a Y chromosome. The testes in individuals with testicular feminization syndrome are often
undescended and are usually removed surgically.
This patient would not possess a uterus (choice B) or an oviduct (choice C). Both of these
structures are derived from the paramesonephric duct, which is suppressed by MIF.
Depressed levels of testosterone (choice D) would not occur in this patient. In fact,
individuals with testicular feminization syndrome have normal or even slightly elevated levels
of testosterone. The development of female external genitalia is the result of defective
androgen receptors, not depressed levels of testosterone.
At which of the following ages does fetal movement first occur?
A. 1 month
B. 2 months
C. 4 months
D. 6 months
E. 7 months
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Explanation:
The correct answer is B. Neuromuscular development is sufficient to allow fetal movement in the
eighth week of life. Other features of week 8 include the first appearance of a thin skin, a
head as large as the rest of the body, forward-looking eyes, appearance of digits on the hands
and feet, appearance of testes and ovaries (but not distinguishable external genitalia), and a
crown-rump length of approximately 30 mm. By the end of the eighth week, nearly all adult
structures have at least begun to develop, and the fetus "looks like a baby".
A neonate is found to have a sacrococcygeal teratoma that contains several different tissue types
resulting from a persistence of the primitive streak. The primitive streak normally gives rise
to which of the following structures?
A. Dorsal root ganglia
B. Lining of the gastrointestinal tract
C. Notochord
D. Spinal cord
E. Thyroid gland
Explanation:
The correct answer is C. The primitive streak is the region of the epiblast through which the
cells that give rise to the notochord and the mesoderm of the embryo pass. The only adult
derivative of the notochord is the nucleus pulposus of the intervertebral disk. The mesoderm
gives rise to many different tissue types including muscle, connective tissue, and blood. This
accounts for the many different tissue types found in a sacrococcygeal teratoma.
The dorsal root ganglia (choice A) are derived from neural crest cells. The neural crest
develops at the time of neurulation, which is a process of infolding of the neural ectoderm that is
induced by the notochord.
The lining of the gastrointestinal tract (choice B) is derived from endoderm. The endoderm also
gives rise to evaginations of the gastrointestinal tract, such as the liver and pancreas. The
smooth muscle and connective tissue of the gastrointestinal tract are derived from mesoderm.
The spinal cord (choice D) is derived from neural ectoderm, which invaginates during
neurulation to form the neural tube. The caudal part of the neural tube forms the spinal cord,
and the rostral part of the neural tube forms the brain.
The thyroid gland (choice E) is derived from endoderm. The endoderm of the floor of the pharynx
evaginates to form the thyroglossal duct, which descends to form the thyroid gland. The adult
site of the evagination of the thyroglossal duct is marked by the foramen cecum on the tongue.
An infant is born with hydrocephalus. CT studies demonstrate herniation of the cerebellum into the
spinal canal. This is an example of which of the following?
A. Arnold-Chiari malformation
B. Dandy-Walker malformation
C. Holoprosencephaly
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D. Lissencephaly
E. Porencephaly
Explanation:
The correct answer is A. Arnold-Chiari malformations are congenital herniations of hindbrain
structures into the spinal canal. In type I Arnold-Chiari malformation, the cerebellar tonsils
herniate into the foramen magnum; in type II, parts of the hindbrain, cerebellar vermis, and
fourth ventricle herniate into the foramen. This second type is commonly associated with
meningomyeloceles. Other neonatal presentations associated with Arnold-Chiari malformation
include hydrocephalus (because of cerebrospinal fluid obstruction) and brainstem dysfunction
(causing stridor and poor swallowing). Milder cases may present later in life with spinal cord
or cerebellar symptoms.
Dandy-Walker malformation (choice B) is associated with hypoplasia of the cerebellum associated
with an enlarged fourth ventricle.
Holoprosencephaly (choice C) is a complex malformation of the brain such that only a single large
monoventricular cerebral hemisphere exists.
Lissencephaly (choice D), or agyria, means "smooth brain" and is a condition characterized by a
lack of hemispheric sulci and severe neurologic impairment.
Porencephaly (choice E) is a severe cleft in the brain that allows the ventricles to
communicate with the subarachnoid space.
During anatomy lab, a medical student notes a fibrous band that runs on the visceral surface of the
liver. It is attached on one end to the inferior vena cava and on the other end to the left
branch of the portal vein. In the embryo, this structure corresponds to the
A. Ductus venosus
B. Ligamentum teres
C. Ligamentum venosum
D. Umbilical arteries
E. Umbilical vein
Explanation:
The correct answer is A. This question could have tricked you if you didn't catch the key
words, "in the embryo." If you read the question too quickly and thought you were going to be
asked to identify the structure described, you probably chose choice C (ligamentum venosum),
since that is indeed the structure in question. However, in the embryo, this fibrous band is
actually the ductus venosus. The ductus venosus is an embryonic vessel that allows blood to
bypass the fetal liver; this prevents the depletion of oxygen and nutrient-rich blood in the
hepatic sinusoids.
The embryonic umbilical vein (choice E) actually becomes the fibrous ligamentum teres (choice B). The
ligamentum teres is located in the free margin of the falciform ligament.
The embryonic umbilical arteries (choice D) become the medial umbilical ligaments.
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A newborn boy does not pass meconium until 48 hours after his birth. Several weeks later his mother
complains that he has not been passing stool regularly. Anorectal manometry reveals increased
internal anal sphincter pressure on rectal distention with a balloon. The patient's disorder
may be attributed to
A. defective recanalization of the colon
B. failure of neural crest cells to migrate into the colonic wall
C. herniation of abdominal contents into the umbilical cord
D. persistence of the proximal end of the yolk stalk
E. presence of a rectourinary fistula
Explanation:
The correct answer is B. The baby boy has Hirschsprung's disease (more common in males), a
diagnosis that should spring to mind in an infant who fails to pass meconium soon after birth
and presents with chronic constipation. Diagnosis is made most easily and most reliably by anal
manometry and/or rectal biopsy. On manometry, internal anal sphincter pressure increases upon
rectal balloon distention in patients with Hirschsprung's disease (normally, pressure decreases
with distention). Rectal biopsy reveals an absence of ganglion cells (which are derived from
neural crest cells) in a portion of the colonic wall. Barium enema would show a transition zone
between the aganglionic area of bowel (narrow) and the region of normal bowel proximal to it
(dilated).
Defective recanalization of the colon (choice A) results in rectal atresia. In this condition,
there is no communication between the rectum and anal canal. Therefore, neither meconium nor stool can
be passed.
Herniation of abdominal contents into the umbilical cord (choice C) describes a congenital
omphalocele. This occurs due to the failure of all or part of the intestines to return into the
abdominal cavity during the tenth week of gestation.
Persistence of the proximal end of the yolk stalk (choice D) results in Meckel's diverticulum,
an ileal outpouching that is more prevalent in males. It usually contains ectopic gastric
mucosa, which can cause ulceration and bleeding. Inflammation of the diverticulum may produce
symptoms and signs similar to appendicitis.
The presence of a rectourinary fistula (choice E) would result in the passage of meconium,
stool, and gas into the urine, a diagnostic finding.
Which of the pharyngeal pouches develops into the palatine tonsil?
A. First
B. Second
C. Third
D. Fourth
E. Fifth
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Explanation:
The correct answer is B. The epithelial lining of the second pharyngeal pouch buds into the
mesenchyme to form the palatine tonsil. Part of the pouch remains in the adult as the tonsillar fossa.
It is important to review the other choices since pharyngeal pouch derivatives are typically
tested on the USMLE Step 1:
The first pharyngeal pouch (choice A) develops into the middle ear cavity and eustachian tube.
The third pharyngeal pouch (choice C) develops into the thymus and the inferior parathyroid glands.
The fourth pharyngeal pouch (choice D) gives rise to the superior parathyroid glands. Recall
that abnormal development of the 3rd and 4th pouches leads to DiGeorge syndrome and results in
hypocalcemia as well as abnormal cellular immunity and consequent susceptibility to viral and
fungal illnesses.
The fifth pharyngeal pouch (choice E) gives rise to the C cells of the thyroid gland. These
cells secrete calcitonin–a hormone that lowers serum calcium.
A 5-year-old child is brought to the emergency room with massive, painless bleeding from the rectum.
Colonoscopy fails to demonstrate a lesion in the colon or anus. Upper endoscopy fails to demonstrate
esophagitis, gastric ulcer, or duodenal ulcer. A [99mTc] technetium scan demonstrates an abnormality in
the lower half of the abdomen. Failure of a normal developmental process involving which of the
following structures is the most likely cause of this child's bleeding?
A. Appendix
B. Cecum
C. Duodenum
D. Ileum
E. Jejunum
Explanation:
The correct answer is D. A Meckel's diverticulum is caused by failure of obliteration of the
vitello-intestinal duct. It is classically located in the distal ileum within 30 cm of the
ileocecal valve, and the structure is a true diverticulum with mucosa, submucosa, and
muscularis propria. Many Meckel's diverticula contain ectopic pancreatic tissue or gastric
mucosa, and the acid production from the gastric mucosa may be sufficient to produce a small
peptic ulcer in adjacent intestinal mucosa. Such small peptic ulcers are occasional sources of
mysterious appendicitis-like pain or intestinal bleeding. Peptic ulceration adjacent to a
Meckel's diverticulum should be suspected in any child who presents with massive, painless
rectal bleeding. Technetium [99mTc] concentrates in gastric mucosa, and the scan in this
patient demonstrated a small amount of ectopic gastric mucosa located in the diverticulum.
Acute appendicitis (choice A) is usually very painful and does not typically cause rectal bleeding.
A lesion of the cecum (choice B) would have been revealed by thorough colonoscopy.
Failure of upper endoscopy to demonstrate a peptic ulcer of the duodenum (choice C) makes duodenal
disease unlikely.
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Theoretically, the jejunum (choice E) could have been the source of the problems, but jejunal
bleeding is uncommon and a Meckel's diverticulum is a much more likely possibility.
A 12-month-old child is diagnosed with an atrial septal defect. What is the most common cause of such a
congenital heart malformation?
A. Failure of formation of the septum primum
B. Failure of formation of the septum secundum
C. Incomplete adhesion between the septum primum and septum secundum
D. Malformation of the membranous interventricular septum
E. Malformation of the muscular interventricular septum
Explanation:
The correct answer is C. The most common form of atrial septal defect is located near the
foramen ovale (not to be confused with a patent foramen ovale, which is of little or no
hemodynamic significance). They result from incomplete adhesion between the septum primum and
the septum secundum during development.
Atrial septal defects less commonly result from failures of formation of the septum primum
(choice A) and septum secundum (choice B).
Malformations of the interventricular septum (choices D and E) cause ventricular septal defects
rather than atrial septal defects.
A newborn male child is noted to have hypospadias. A complete evaluation determines that the child has
no other genitourinary anomalies. Nonetheless, hypospadias repair will be performed to prevent
which of the following possible sequelae?
A. Bladder exstrophy
B. Hydrocele
C. Phimosis
D. Urachal cysts
E. Urinary tract infection
Explanation:
The correct answer is E. Hypospadias, which is congenital displacement of the urethral opening
onto the ventral surface (underside) of the penis, is due to malformation of the urethral
groove and canal. Hypospadias frequently accompanies other genitourinary anomalies, especially
cryptorchidism. Isolated hypospadias is repaired because the abnormal opening is often
constricted, leading to urinary retention and ascending urinary tract infections. Another
important consequence of hypospadias is sterility, which occurs if the opening is too close to
the base of the penis to permit normal ejaculation. Although hypospadias can occur in
isolation, it is strongly associated with other urogenital anomalies.
Bladder exstrophy (choice A) is a completely unrelated congenital malformation in which the
abdominal wall and anterior bladder wall form incompletely, and the bladder mucosa is exposed to the
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environment.
Hydrocele (choice B) is a serous accumulation in the tunica vaginalis, often producing a
readily apparent scrotal mass. Hydrocele usually arises without any obvious cause.
Phimosis (choice C) is the condition in which the foreskin cannot be retracted over the head of
the penis. It is usually either a congenital malformation or a consequence of scarring.
Urachal cysts (choice D) are produced by areas of persistent urachal epithelium, which may be present
anywhere between the bladder and the umbilicus.
A 25-year-old female presents to her obstetrician after taking a home pregnancy test with a positive
result. She states that twins run in her family and would like an ultrasound to determine if
she has a twin pregnancy. Radiographic studies confirm that the embryo has split at the
blastocyst stage. Splitting of the embryo at the blastocyst stage results in which of the
following?
A. Conjoined twins
B. Dizygotic twins
C. Fraternal twins
D. Monozygotic twins
E. Single gestation
Explanation:
The correct answer is D. Monozygotic twins, or identical twins, develop from a single
fertilized egg that subsequently splits during either the blastomere or blastocyst stage. It is
more common in the blastocyst stage at the end of the first week. This results in two inner
cell masses in the same blastocyst cavity. They usually develop a common placenta and chorionic
cavities but separate amniotic cavities. Splitting in the second week usually results in shared
amniotic cavities as well.
Conjoined twins (choice A), or Siamese twins, result from incomplete splitting of the embryo.
Dizygotic twins (choice B) and fraternal twins (choice C) are the same, and are the most common
type of twins. They share the same genetic relatedness as do siblings of separate pregnancies.
This type of twinning occurs because of simultaneous double ovulation followed by fertilization
by two sperm. They each develop their own placenta and membranes.
A single gestation (choice E), and a single birth, is the result of a single fertilization
without any splitting of the embryo.
On physical examination of a 7-year-old boy, the child's upper body appears much more developed than his
lower body. Blood pressure in the upper extremities exceeds that of the lower extremities. On cardiac
examination, there is a midsystolic murmur over the anterior chest and back. The child's lower
extremities are cold, and femoral pulses are absent. The part of the vascular system that is affected in
this disorder is derived from which of the following embryologic structures?
A. Bulbus cordis
B. Ductus arteriosus
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C. Left horn of sinus venosus
D. Right common cardinal vein
E. Right horn of sinus venosus
F. Third, fourth, and sixth aortic arches
Explanation:
The correct answer is F. This patient has coarctation of the aorta (constriction of the
ascending aorta), which is suggested by a midsystolic murmur over the anterior part of the
chest and back, hypertension in the upper extremities, and absent or delayed pulsations in the
femoral arteries. The upper extremities and thorax may be more developed than the lower
extremities. Patients with coarctation of the aorta may experience symptoms such as cold
extremities as a result of tissue ischemia.The truncus arteriosus gives rise to the proximal
portions of the ascending aorta and the pulmonary trunk. The third, 4th, and 6th aortic arches
and the right and left dorsal aortae contribute to the remainder of the aorta.
The bulbus cordis (choice A) gives rise to the right ventricle and the aortic outflow tract.
98% of cases of coarctation of the aorta take place immediately distal to the offshoot of the
left subclavian artery, close to the junction of the ductus arteriosus (choice B) with the aorta.
The left horn of the sinus venosus (choice C) gives rise to the coronary sinus.
The right common cardinal vein (choice D) gives rise to the superior vena cava.
The right horn of the sinus venosus (choice E) gives rise to the smooth part of the right atrium.