Sample Questions from MicrobeLibrary’s
Critical Thinking Question Bank (CTQB)
ASMCUE 2012
Becky Buxton
University of Utah
Salt Lake City, UT
Question set #1
Concept(s): Microbiology Skills; Systems
Category: Laboratory Test
For each of the following urinalysis (UA) and urine culture results, indicate whether or not the patient has a
bacterial UTI, and the probable causative agent.
(All samples are "clean catch," mid-stream samples)
UA
WBC's
many
RBC's
occasional
Bacteria
many
Protein
slight
NO3
pos
(Patient is a 25 year old female.)
Culture
1,000 cfu/ml Lactobacillus sp.
5,000 cfu/ml diphtheroids
>100,000 cfu/ml E. coli
1. Does the person have a urinary tract infection?
A(1). yes
B(2). no
2. What is the probable causative agent?
A(1). Lactobacillus sp.
B.(2) Diphtheroids
C(3). E. coli
D(4). None (Results indicate vaginal contamination.)
Author’s Comment: This culture indicates slight vaginal contamination, but an overwhelming
number of E. coli and presence of WBC’s indicate infection. The student should be familiar with
quantitative culture of Urine for clinical diagnosis and be able to interpret the possible role of
contaminating organisms.
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Question #2
Concept(s): Microbiology Skills; Systems
Category: Laboratory Test
A direct smear of an abscess exudate showed 4+ white blood cells s and 4+ gram-positive cocci. A
Staphylococcus species was recovered from the culture the next day. A tube coagulase test was set up and
incubated at 37C. When read the next day, the coagulase test was liquid. The colonies on the sheep blood agar
plate, held at room temperature were hemolytic and slightly yellowish (“creamy”). How would you deal with these
findings?
A(1). Report, “Coagulase-negative Staphylococcus” species (probable skin contamination)
B(2). Report Staphylococcus aureus
C(3). Repeat the coagulase test incubated at 37C and read it in four hours.
D(4). Repeat the coagulase test and incubate it overnight at room temperature.
Author’s Comment: The presence of WBC’s and large numbers of organisms in the direct sample suggest
infection. The colony morphology is consistent with S. aureus. Many strains of S. aureus produce a fibrinolysin
that can dissolve the clot on prolonged incubation. The clot should form within four hours, but may dissolve before
24 hours.
Question #3
Concept(s): Microbiology Skills; Systems
Category: Laboratory Test
Evaluate the above sputum sample for culture.
A(1). Acceptable for culture
B(2). Not acceptable for culture
C(3). Not enough information to say
Author’s Comment: The presence of squamous epithelial cells indicates contamination with oral flora; therefore,
the sample would not accurately represent the infectious agent of a lower respiratory infection.
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Question #4
Concept(s): Microbiology Skills; Systems
Category: Laboratory Test
If the following organism was recovered from the uterine cervix of an 18-year-old female, does it confirm the
presence of a sexually transmitted disease?
Gram Stain: gram-negative diplococcus
Nutrient Agar: good growth
Oxidase: positive
ONPG: negative
CTA Sugars:
Glucose: acid
Maltose: alkaline
Lactose: alkaline
Sucrose: alkaline
A(1). Yes. These results are consistent with Neisseria gonorrhoeae
B(2). No. These results are consistent with Neisseria lactamica, a common vaginal tract colonizer
C(3). No. These results are more consistent with a short gram-negative rod masquerading as a
diplococcus.
D(4). Unable to determine from these data.
Author’s Comment: N. gonorrhoeae is a fastidious organism that cannot grow on nutrient agar. Occasionally,
short, oxidase-positive, gram-negative rods may colonize the vaginal tract and appear similar to Neisseria sp.
The student should be acquainted with the routine culturing of clinical samples for bacterial pathogens. This
exercise helps enforce the fastidious nature of N. gonorrhoeae and the possibility of contaminating commensal
organisms with similar biochemical profiles.
Erica Suchman
Colorado State University
Ft. Collins, CO
Question #1
Concept(s): Structure and Function
Category: Bacteria
What components of the peptidoglycan can be altered to change the thickness of a Gram negative peptidoglycan
layer?
A(1). The number of NAG and NAM sheets
B(2). The type of amino acids in the interbridge
C(3). The number of amino acids in the interbridge
D(4). A & B
E(5). A, B & C
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Author’s Comment: This requires a student to understand the chemical makeup of a Gram negative peptidoglycan
layer and how this structure can vary in different Gram negative organism. Based on their knowledge of the
peptidoglycan structure they must be able to predict which parts can be altered.
Question #2
Concept(s): Structure and Function
Category: Bacteria
What components of the peptidoglycan can be altered to change the thickness of a Gram positive peptidoglycan
layer?
A(1). The number of NAG and NAM sheets
B(2). The type of amino acids in the interbridge
C(3). The number of amino acids in the interbridge
D(4). A & B
E(5). A, B & C
Author’s Comment: This requires a student to understand the chemical makeup of a Gram positive peptidoglycan
layer and how this structure can vary in different Gram positive organism. Based on their knowledge of the
peptidoglycan structure they must be able to predict which parts can be altered.
Question #3
Concept(s): Information Flow
Category: Bacteria
You perform an interrupted mating Hfr gene mapping experiment and get the following data. What is the order of
the genes relative to the F factor for this bacterium?
Time conjugating
0 sec
30 sec
1 min
2 min
3 min
Recipient traits after given amount of time
s
s
R
His , Lys , Strep , Tet , and Pen
R
s
R
His , Lys , Stre , Tet , and Pen
+
R
s
R
His , Lys , Strep , Tet , and Pen
+
+
R
s
R
His , Lys , Strep , Tet , and Pen
+
+
R
R
R
His , Lys , Strep , Tet , and Pen
A(1). His Lys Strep Tet Pen
B(2). Strep, His, Lys, Tet , Pen
C(3). Pen, Tet Strep, Lys, His
D(4) Pen, Tet, Lys, His, Strep
Author’s Comment: This question requires students to understand how Hfr mapping works and how to predict the
order of genes based on data.
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Question #4
Concept(s): Information Flow
Category: Bacteria
In the Griffith experiments what are the possible fates of exogenous chromosomal DNA taken up by an R cell
from the S cell if the R cell becomes an S?
A(1). Integration
B(2). Degradation
C(3). Independent replication
D(4). A & B
E(5). A, B & C
Author’s Comment: This requires the student to understand the possible fates of exogenous DNA in a cell and
determine which fates will lead to the outcome seen. If the capsule gene is degraded the cell will remain R. The
DNA taken up was not in a plasmid, it was a chromosomal fragment so it could not independently replicate.
Samantha Elliott
St. Mary’s College of Maryland
St. Mary's City, MD
Question #1
Concept(s): Structure and Function; Systems
Category: Immunology
Many newborns do not show any signs of immune system dysfunction until 3-6 months after birth, particularly if
the mother does not breastfeed. Why might this be the case?
A(1). Maternal immune memory cells only live for a few months
B(2). Only maternal immune effector cells are passed through the placenta
C(3). The newborn does not make antibodies until after birth
D(4). The maternal immune contribution constitutes passive immunization
E(5). Only certain antibodies pass through the placenta
Author comments: This question tests concepts of the ability of immune cells and antibodies to cross the placental
barrier, and when during development a fetus begins to create its own immune response. Actual immune cells do
not cross the placental barrier, and only certain antibodies (IgG) can cross the placenta, due to size restrictions.
IgG antibodies are long-lived, allowing for months of passive immunization, which will continue if the mother
breastfeeds. The developing fetus begins to create antibodies after birth (which makes C a true statement and
major distractor), but maternal passive immunity allows for potential immune system dysfunction to be masked
until such time as maternal antibodies are degraded. Without passive immunity, immune dysfunction of the
newborn would be apparent much earlier. E is also a true statement, but even though only IgG passes across the
placental barrier, passive immunity is successful.
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Question #2
Concept(s): Structure and Function; Systems
Category: Immunology
Pine pollen is often blamed as a culprit of spring allergies. However, pine pollen’s large size makes it a poor
allergen (antigen that causes allergies). Based upon what you know about antigens, what is the best explanation
for this phenomenon?
A(1). Pine pollen is too large to be easily spread in the air and therefore is not a good allergen.
B(2). Pine pollen is too large to be processed and presented on MHC.
C(3). Pine pollen is not a protein and therefore cannot be an allergen.
D(4). Pine pollen is too close in size to human proteins and therefore is tolerated by the immune system.
E(5). Pine pollen’s size makes it too complex to be an allergen.
Author comments: This question tests students’ ability to comprehend the characteristics of antigens, and also
connect that to how antigens are presented to the immune system. Antigen presentation involves chopping up
proteins to smaller bits, ruling out B and D. Because B cells recognize both protein and non-protein antigens, C
can be eliminated. The more complex an antigen, often the better it is, thus ruling out E. Antigens can be made
up of many things (proteins, carbohydrates, lipids) and the primary mechanism for detecting allergens is IgE
antibodies. A is the correct answer, because the pollen often doesn’t really travel through the air enough to be
inhaled—good allergens are small and travel long distances. It is a major misconception that pine pollen causes
allergies, because other pollens that occur at the same time are the primary culprits. Pine pollen does act as an
irritant, but very few people are actually allergic to it.
Question #3
Concept(s): Structure and Function; Systems
Category: Immunology
Someone who has exclusively endogenous antigens expressed on MHC II must have a dysfunction of which of
the following proteins?
A(1). HLA-DM
B(2). CLIP
C(3). Invariant chain (Ii)
D(4). Peptide transporter
E(5). Proteasome
Author’s comments: This question tests student understanding of antigen processing and presentation. The
possible answers constitute portions of both MHC I (D and E) and MHC II (A through C) pathways. MHC I
presents primarily endogenous antigens, while MHC II presents primarily exogenous antigens. Crosspresentation allows for expression of exogenous antigens on MHC I. Invariant chain blocks peptide binding to
MHC II in the ER, thus allowing MHC II to leave the ER without an associated peptide, thus this is the correct
answer. Invariant chain is degraded to CLIP and HLA-DM facilitates the swap of CLIP for an exogenous antigen
within the phagolysosome, thus making both A and B major distractors for this question.
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Question #4
Concept(s): Structure and Function; Systems
Category: Immunology
Some bacteria have the enzymes superoxide dismutase and catalase, which allow the bacteria to convert
hydrogen peroxide into water. Which of the following immune responses would be most impaired by this?
A(1). Complement
B(2). CD4 T cells
C(3). B cells
D(4). Macrophages
E(5). NK cells
Author comments: Reactive oxygen species are the primary way in which macrophages and neutrophils kill
pathogens. NK cells release granules, B cells secrete antibodies, and CD4 T cells secrete cytokines.
Complement proteins attack pathogens directly, or cause inflammation.
Question #5
Concept(s): Structure and Function; Systems
Category: Immunology
Reactivation of Varicella zoster, the virus that causes chickenpox, causes shingles. While painful, shingles lasts a
few weeks, then goes away. Development of shingles is most likely a result of:
A(1). New exposure to the chickenpox virus from an infected person.
B(2). Failure of innate immunity to respond during the initial chickenpox infection.
C(3). Failure of adaptive immunity to respond during the initial chickenpox infection.
D(4). Failure to establish immune memory during the initial chickenpox infection.
E(5). Failure of immune memory to keep the virus contained.
Author comment: This question requires students to recognize that a typical immune response has occurred in
the original infection (chicken pox) and that the re-emergence of disease (shingles) is a potential defect in
immune memory maintenance. While the primary mechanism of shingles development is unknown, it is usually
seen in patients with suppressed immune systems.
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