MICROBIOLOGY ASCP EXAM 2023 200 ACTUAL QUESTIONS
AND CORRECT ANSWERS WITH RATIONALE LATEST 2026
ALREADY GRADED A+
Each entry has a unique combination of patient presentation, culture
characteristics, biochemical reactions, or test interpretation. While certain
high-yield organisms (such as *Staphylococcus aureus*, *Escherichia coli*,
*Pseudomonas aeruginosa*, *Streptococcus pneumoniae*, and *Neisseria
meningitidis*) appear frequently, they are tested through different clinical
scenarios, different media selections, or unique phenotypic traits like pigment
production, hemolysis patterns, or fermentation profiles. The answer choices
also vary. Therefore, every question is structurally and contextually distinct,
providing comprehensive, non-redundant coverage of the ASCP microbiology
content blueprint.
1. A stool culture from a patient with suspected gastrointestinal infection grows an
organism with the following characteristics: colorless colonies on MacConkey
agar, yellow-orange colonies on Hektoen agar, acid slant/acid butt on TSI with no
gas or H₂S, and a positive urease test. What is the most likely identification?
a) Shigella sonnei
b) Vibrio parahaemolyticus
c) Yersinia enterocolitica
d) Campylobacter jejuni
Correct Answer: c) Yersinia enterocolitica
Rationale: Yersinia enterocolitica is a classic enteric pathogen that can mimic
appendicitis. Its biochemical profile includes being a non-lactose fermenter
(colorless on MacConkey), producing yellow-orange colonies on Hektoen enteric
agar, giving an acid/acid (A/A) reaction on TSI without H₂S production, and being
strongly urease positive. Shigella is nonmotile and urease negative, Campylobacter
requires special microaerophilic conditions and does not grow on routine enteric
media, and Vibrio would typically be oxidase positive and grow on TCBS agar.
2. Which test is most appropriate to presumptively identify group A streptococci
from a throat culture?
a) PYR test
,b) Bacitracin susceptibility
c) Optochin susceptibility
d) Bile solubility
Correct Answer: b) Bacitracin susceptibility
Rationale: The bacitracin susceptibility test is used for presumptive ident ification
of Streptococcus pyogenes (group A strep). A zone of inhibition around a 0.04 -unit
bacitracin disk indicates susceptibility, which is characteristic of group A. PYR is
also positive for group A but is more often used for enterococci and group A s trep
in rapid testing; bacitracin is the classic screening test. Optochin is used for
Streptococcus pneumoniae and bile solubility is also for pneumococci.
3. An organism isolated from a respiratory specimen is oxidase positive, catalase
positive, indole negative, and grows on MacConkey agar. It produces a distinct
fruity odor. What is the most likely organism?
a) Pseudomonas aeruginosa
b) Burkholderia cepacia
c) Stenotrophomonas maltophilia
d) Acinetobacter baumannii
Correct Answer: a) Pseudomonas aeruginosa
Rationale: Pseudomonas aeruginosa is oxidase positive, catalase positive, grows on
MacConkey, and produces a characteristic grape-like or fruity odor due to 2-
aminoacetophenone. It is indole negative. Burkholderia is oxidase variable and
often positive for lysine decarboxylase; Stenotrophomonas is oxidase negative and
has a different biochemical profile; Acinetobacter is oxidase negative and does not
produce the typical odor.
4. A blood culture from a neutropenic patient grows filamentous, gram-positive,
branching rods that are weakly acid-fast. What is the most likely organism?
a) Actinomyces israelii
b) Nocardia asteroides
c) Mycobacterium tuberculosis
d) Propionibacterium acnes
Correct Answer: b) Nocardia asteroides
Rationale: Nocardia species are weakly acid-fast, gram-positive branching
filamentous rods and are opportunistic pathogens in immunocompromised patients.
Actinomyces is not acid-fast and grows anaerobically; Mycobacterium tuberculosis
is strongly acid-fast and does not branch; Propionibacterium is a non-branching
anaerobic diphtheroid.
,5. Which of the following media is used to selectively isolate Neisseria
gonorrhoeae from clinical specimens?
a) Thayer-Martin agar
b) MacConkey agar
c) EMB agar
d) Hektoen enteric agar
Correct Answer: a) Thayer-Martin agar
Rationale: Thayer-Martin agar is a selective medium for pathogenic Neisseria
species. It contains antibiotics (vancomycin, colistin, nystatin, and trimethoprim) to
inhibit normal flora while allowing N. gonorrhoeae and N. meningitidis to grow.
MacConkey and EMB are for gram-negative enteric organisms; Hektoen is for
enteric pathogens.
6. A patient presents with a painful ulcerated lesion on the hand after handling fish.
Culture grows a curved, gram-negative rod on TCBS agar with yellow colonies.
What is the most likely pathogen?
a) Vibrio vulnificus
b) Vibrio cholerae
c) Vibrio parahaemolyticus
d) Aeromonas hydrophila
Correct Answer: c) Vibrio parahaemolyticus
Rationale: Vibrio parahaemolyticus is associated with wound infections and
gastroenteritis from seafood exposure. On TCBS agar, it produces green or blue-
green colonies typically, but some strains can appear yellow due to sucrose
fermentation. However, V. cholerae is sucrose positive (yellow) and V. vulnificus
is also sucrose positive but more associated with septicemia; V. parahaemolyticus
is the classic cause of wound infections from marine environments. Aeromonas
does not grow well on TCBS.
7. An organism is catalase negative, esculin positive, and grows in 6.5% NaCl
broth. It is bile esculin positive and PYR positive. What is the organism?
a) Streptococcus pneumoniae
b) Enterococcus faecalis
c) Streptococcus pyogenes
d) Streptococcus agalactiae
Correct Answer: b) Enterococcus faecalis
Rationale: Enterococci are catalase negative, esculin positive, bile esculin positive,
grow in 6.5% NaCl, and are PYR positive. Streptococcus pneumoniae is bile
soluble and optochin susceptible; S. pyogenes is PYR positive but does not grow in
, 6.5% NaCl or hydrolyze esculin; S. agalactiae is CAMP positive and does not
grow in 6.5% NaCl.
8. Which of the following organisms is most likely to produce a positive oxidase
test?
a) Escherichia coli
b) Salmonella enterica
c) Neisseria meningitidis
d) Shigella flexneri
Correct Answer: c) Neisseria meningitidis
Rationale: The oxidase test detects cytochrome c oxidase. Neisseria species are
oxidase positive. Escherichia, Salmonella, and Shigella are all oxidase negative as
they are members of the Enterobacteriaceae family.
9. A urine culture grows 100,000 CFU/mL of a lactose-fermenting, indole-positive,
citrate-negative, motile organism. What is the most likely identification?
a) Klebsiella pneumoniae
b) Enterobacter cloacae
c) Escherichia coli
d) Proteus mirabilis
Correct Answer: c) Escherichia coli
Rationale: E. coli is the most common cause of urinary tract infections. It is lactose
fermenter, indole positive, citrate negative, and motile. Klebsiella is nonmotile and
indole negative; Enterobacter is citrate positive and indole negative; Proteus is
lactose negative, indole negative, and urease positive.
10. A gram-positive coccus is catalase positive, coagulase negative, and
novobiocin resistant. What is the organism?
a) Staphylococcus aureus
b) Staphylococcus saprophyticus
c) Staphylococcus epidermidis
d) Streptococcus agalactiae
Correct Answer: b) Staphylococcus saprophyticus
Rationale: Staphylococcus saprophyticus is a coagulase-negative staphylococcus
that is novobiocin resistant. It is a common cause of urinary tract infections in
young women. S. aureus is coagulase positive; S. epidermidis is novobiocin
susceptible; Streptococcus is catalase negative.
11. An anaerobic culture from an aspirated abscess grows gram-negative bacilli
that are resistant to kanamycin, vancomycin, and colistin. What is the genus?
AND CORRECT ANSWERS WITH RATIONALE LATEST 2026
ALREADY GRADED A+
Each entry has a unique combination of patient presentation, culture
characteristics, biochemical reactions, or test interpretation. While certain
high-yield organisms (such as *Staphylococcus aureus*, *Escherichia coli*,
*Pseudomonas aeruginosa*, *Streptococcus pneumoniae*, and *Neisseria
meningitidis*) appear frequently, they are tested through different clinical
scenarios, different media selections, or unique phenotypic traits like pigment
production, hemolysis patterns, or fermentation profiles. The answer choices
also vary. Therefore, every question is structurally and contextually distinct,
providing comprehensive, non-redundant coverage of the ASCP microbiology
content blueprint.
1. A stool culture from a patient with suspected gastrointestinal infection grows an
organism with the following characteristics: colorless colonies on MacConkey
agar, yellow-orange colonies on Hektoen agar, acid slant/acid butt on TSI with no
gas or H₂S, and a positive urease test. What is the most likely identification?
a) Shigella sonnei
b) Vibrio parahaemolyticus
c) Yersinia enterocolitica
d) Campylobacter jejuni
Correct Answer: c) Yersinia enterocolitica
Rationale: Yersinia enterocolitica is a classic enteric pathogen that can mimic
appendicitis. Its biochemical profile includes being a non-lactose fermenter
(colorless on MacConkey), producing yellow-orange colonies on Hektoen enteric
agar, giving an acid/acid (A/A) reaction on TSI without H₂S production, and being
strongly urease positive. Shigella is nonmotile and urease negative, Campylobacter
requires special microaerophilic conditions and does not grow on routine enteric
media, and Vibrio would typically be oxidase positive and grow on TCBS agar.
2. Which test is most appropriate to presumptively identify group A streptococci
from a throat culture?
a) PYR test
,b) Bacitracin susceptibility
c) Optochin susceptibility
d) Bile solubility
Correct Answer: b) Bacitracin susceptibility
Rationale: The bacitracin susceptibility test is used for presumptive ident ification
of Streptococcus pyogenes (group A strep). A zone of inhibition around a 0.04 -unit
bacitracin disk indicates susceptibility, which is characteristic of group A. PYR is
also positive for group A but is more often used for enterococci and group A s trep
in rapid testing; bacitracin is the classic screening test. Optochin is used for
Streptococcus pneumoniae and bile solubility is also for pneumococci.
3. An organism isolated from a respiratory specimen is oxidase positive, catalase
positive, indole negative, and grows on MacConkey agar. It produces a distinct
fruity odor. What is the most likely organism?
a) Pseudomonas aeruginosa
b) Burkholderia cepacia
c) Stenotrophomonas maltophilia
d) Acinetobacter baumannii
Correct Answer: a) Pseudomonas aeruginosa
Rationale: Pseudomonas aeruginosa is oxidase positive, catalase positive, grows on
MacConkey, and produces a characteristic grape-like or fruity odor due to 2-
aminoacetophenone. It is indole negative. Burkholderia is oxidase variable and
often positive for lysine decarboxylase; Stenotrophomonas is oxidase negative and
has a different biochemical profile; Acinetobacter is oxidase negative and does not
produce the typical odor.
4. A blood culture from a neutropenic patient grows filamentous, gram-positive,
branching rods that are weakly acid-fast. What is the most likely organism?
a) Actinomyces israelii
b) Nocardia asteroides
c) Mycobacterium tuberculosis
d) Propionibacterium acnes
Correct Answer: b) Nocardia asteroides
Rationale: Nocardia species are weakly acid-fast, gram-positive branching
filamentous rods and are opportunistic pathogens in immunocompromised patients.
Actinomyces is not acid-fast and grows anaerobically; Mycobacterium tuberculosis
is strongly acid-fast and does not branch; Propionibacterium is a non-branching
anaerobic diphtheroid.
,5. Which of the following media is used to selectively isolate Neisseria
gonorrhoeae from clinical specimens?
a) Thayer-Martin agar
b) MacConkey agar
c) EMB agar
d) Hektoen enteric agar
Correct Answer: a) Thayer-Martin agar
Rationale: Thayer-Martin agar is a selective medium for pathogenic Neisseria
species. It contains antibiotics (vancomycin, colistin, nystatin, and trimethoprim) to
inhibit normal flora while allowing N. gonorrhoeae and N. meningitidis to grow.
MacConkey and EMB are for gram-negative enteric organisms; Hektoen is for
enteric pathogens.
6. A patient presents with a painful ulcerated lesion on the hand after handling fish.
Culture grows a curved, gram-negative rod on TCBS agar with yellow colonies.
What is the most likely pathogen?
a) Vibrio vulnificus
b) Vibrio cholerae
c) Vibrio parahaemolyticus
d) Aeromonas hydrophila
Correct Answer: c) Vibrio parahaemolyticus
Rationale: Vibrio parahaemolyticus is associated with wound infections and
gastroenteritis from seafood exposure. On TCBS agar, it produces green or blue-
green colonies typically, but some strains can appear yellow due to sucrose
fermentation. However, V. cholerae is sucrose positive (yellow) and V. vulnificus
is also sucrose positive but more associated with septicemia; V. parahaemolyticus
is the classic cause of wound infections from marine environments. Aeromonas
does not grow well on TCBS.
7. An organism is catalase negative, esculin positive, and grows in 6.5% NaCl
broth. It is bile esculin positive and PYR positive. What is the organism?
a) Streptococcus pneumoniae
b) Enterococcus faecalis
c) Streptococcus pyogenes
d) Streptococcus agalactiae
Correct Answer: b) Enterococcus faecalis
Rationale: Enterococci are catalase negative, esculin positive, bile esculin positive,
grow in 6.5% NaCl, and are PYR positive. Streptococcus pneumoniae is bile
soluble and optochin susceptible; S. pyogenes is PYR positive but does not grow in
, 6.5% NaCl or hydrolyze esculin; S. agalactiae is CAMP positive and does not
grow in 6.5% NaCl.
8. Which of the following organisms is most likely to produce a positive oxidase
test?
a) Escherichia coli
b) Salmonella enterica
c) Neisseria meningitidis
d) Shigella flexneri
Correct Answer: c) Neisseria meningitidis
Rationale: The oxidase test detects cytochrome c oxidase. Neisseria species are
oxidase positive. Escherichia, Salmonella, and Shigella are all oxidase negative as
they are members of the Enterobacteriaceae family.
9. A urine culture grows 100,000 CFU/mL of a lactose-fermenting, indole-positive,
citrate-negative, motile organism. What is the most likely identification?
a) Klebsiella pneumoniae
b) Enterobacter cloacae
c) Escherichia coli
d) Proteus mirabilis
Correct Answer: c) Escherichia coli
Rationale: E. coli is the most common cause of urinary tract infections. It is lactose
fermenter, indole positive, citrate negative, and motile. Klebsiella is nonmotile and
indole negative; Enterobacter is citrate positive and indole negative; Proteus is
lactose negative, indole negative, and urease positive.
10. A gram-positive coccus is catalase positive, coagulase negative, and
novobiocin resistant. What is the organism?
a) Staphylococcus aureus
b) Staphylococcus saprophyticus
c) Staphylococcus epidermidis
d) Streptococcus agalactiae
Correct Answer: b) Staphylococcus saprophyticus
Rationale: Staphylococcus saprophyticus is a coagulase-negative staphylococcus
that is novobiocin resistant. It is a common cause of urinary tract infections in
young women. S. aureus is coagulase positive; S. epidermidis is novobiocin
susceptible; Streptococcus is catalase negative.
11. An anaerobic culture from an aspirated abscess grows gram-negative bacilli
that are resistant to kanamycin, vancomycin, and colistin. What is the genus?
