Microbiology for COMLEX Level 1 –
Comprehensive Exam Review |
2026–2027
Description: This comprehensive question bank contains 200 high-yield multiple-
choice questions covering bacteriology, virology, mycology, parasitology,
immunology, and antimicrobial pharmacology for COMLEX Level 1 preparation.
Keywords: COMLEX Level 1, Microbiology, Bacteriology, Virology, Mycology,
Parasitology, Immunology, Antimicrobials, USMLE, Osteopathic Medicine, High-Yield
Review
Section 1: Bacteriology – General Principles & Cell Structure
(Questions 1–25)
Q1. A 24-year-old medical student develops a skin abscess after a needlestick injury in
the anatomy lab. Gram stain reveals gram-positive cocci in clusters that are catalase-
positive and coagulase-positive. Which organism is most likely responsible?
A. Staphylococcus epidermidis
B. Staphylococcus aureus ✅
C. Streptococcus pyogenes
D. Enterococcus faecalis
,Q2. A gram-negative diplococcus is isolated from the cerebrospinal fluid of a college
student with fever, neck stiffness, and a petechial rash. The organism ferments
glucose and maltose but not lactose. What is the most likely causative agent?
A. Neisseria gonorrhoeae
B. Neisseria meningitidis ✅
C. Haemophilus influenzae
D. Streptococcus pneumoniae
Q3. A patient with gastric ulcers undergoes endoscopy, and a curved gram-negative
rod is identified that is urease-positive and microaerophilic. Which organism is most
likely?
A. Campylobacter jejuni
B. Vibrio cholerae
C. Helicobacter pylori ✅
D. Salmonella enterica
Q4. A gram-positive rod that is spore-forming, catalase-positive, and motile is
isolated from a patient with food poisoning (reheated rice). What is the most likely
organism?
A. Bacillus cereus ✅
B. Clostridium perfringens
C. Listeria monocytogenes
D. Corynebacterium diphtheriae
,Q5. A gram-negative rod that is oxidase-positive, lactose-fermenting, and produces
a blue-green pigment is isolated from a burn wound. What is the most likely
organism?
A. Escherichia coli
B. Pseudomonas aeruginosa ✅
C. Klebsiella pneumoniae
D. Proteus mirabilis
Q6. A gram-positive coccus in pairs and chains is catalase-negative, bacitracin-
sensitive, and optochin-sensitive. It is most likely:
A. Streptococcus pneumoniae ✅
B. Streptococcus pyogenes
C. Streptococcus agalactiae
D. Viridans streptococci
Q7. A gram-negative rod that is non-lactose fermenting, oxidase-negative, and
produces H₂S on Salmonella-Shigella agar is most likely:
A. Shigella flexneri
B. Salmonella enterica ✅
C. Escherichia coli
D. Yersinia enterocolitica
, Q8. A gram-positive rod that is non-spore-forming, catalase-positive, and tumbling
motility at 25°C is isolated from a patient with meningitis. What is the most likely
organism?
A. Listeria monocytogenes ✅
B. Corynebacterium diphtheriae
C. Bacillus anthracis
D. Clostridium botulinum
Q9. A gram-negative coccobacillus that requires X and V factors for growth is most
likely:
A. Bordetella pertussis
B. Haemophilus influenzae ✅
C. Brucella abortus
D. Francisella tularensis
Q10. A gram-positive rod with metachromatic granules and Chinese-letter
arrangement is isolated from a patient with pharyngitis and pseudomembrane. What
is the most likely organism?
A. Corynebacterium diphtheriae ✅
B. Bacillus cereus
C. Clostridium tetani
D. Listeria monocytogenes
Comprehensive Exam Review |
2026–2027
Description: This comprehensive question bank contains 200 high-yield multiple-
choice questions covering bacteriology, virology, mycology, parasitology,
immunology, and antimicrobial pharmacology for COMLEX Level 1 preparation.
Keywords: COMLEX Level 1, Microbiology, Bacteriology, Virology, Mycology,
Parasitology, Immunology, Antimicrobials, USMLE, Osteopathic Medicine, High-Yield
Review
Section 1: Bacteriology – General Principles & Cell Structure
(Questions 1–25)
Q1. A 24-year-old medical student develops a skin abscess after a needlestick injury in
the anatomy lab. Gram stain reveals gram-positive cocci in clusters that are catalase-
positive and coagulase-positive. Which organism is most likely responsible?
A. Staphylococcus epidermidis
B. Staphylococcus aureus ✅
C. Streptococcus pyogenes
D. Enterococcus faecalis
,Q2. A gram-negative diplococcus is isolated from the cerebrospinal fluid of a college
student with fever, neck stiffness, and a petechial rash. The organism ferments
glucose and maltose but not lactose. What is the most likely causative agent?
A. Neisseria gonorrhoeae
B. Neisseria meningitidis ✅
C. Haemophilus influenzae
D. Streptococcus pneumoniae
Q3. A patient with gastric ulcers undergoes endoscopy, and a curved gram-negative
rod is identified that is urease-positive and microaerophilic. Which organism is most
likely?
A. Campylobacter jejuni
B. Vibrio cholerae
C. Helicobacter pylori ✅
D. Salmonella enterica
Q4. A gram-positive rod that is spore-forming, catalase-positive, and motile is
isolated from a patient with food poisoning (reheated rice). What is the most likely
organism?
A. Bacillus cereus ✅
B. Clostridium perfringens
C. Listeria monocytogenes
D. Corynebacterium diphtheriae
,Q5. A gram-negative rod that is oxidase-positive, lactose-fermenting, and produces
a blue-green pigment is isolated from a burn wound. What is the most likely
organism?
A. Escherichia coli
B. Pseudomonas aeruginosa ✅
C. Klebsiella pneumoniae
D. Proteus mirabilis
Q6. A gram-positive coccus in pairs and chains is catalase-negative, bacitracin-
sensitive, and optochin-sensitive. It is most likely:
A. Streptococcus pneumoniae ✅
B. Streptococcus pyogenes
C. Streptococcus agalactiae
D. Viridans streptococci
Q7. A gram-negative rod that is non-lactose fermenting, oxidase-negative, and
produces H₂S on Salmonella-Shigella agar is most likely:
A. Shigella flexneri
B. Salmonella enterica ✅
C. Escherichia coli
D. Yersinia enterocolitica
, Q8. A gram-positive rod that is non-spore-forming, catalase-positive, and tumbling
motility at 25°C is isolated from a patient with meningitis. What is the most likely
organism?
A. Listeria monocytogenes ✅
B. Corynebacterium diphtheriae
C. Bacillus anthracis
D. Clostridium botulinum
Q9. A gram-negative coccobacillus that requires X and V factors for growth is most
likely:
A. Bordetella pertussis
B. Haemophilus influenzae ✅
C. Brucella abortus
D. Francisella tularensis
Q10. A gram-positive rod with metachromatic granules and Chinese-letter
arrangement is isolated from a patient with pharyngitis and pseudomembrane. What
is the most likely organism?
A. Corynebacterium diphtheriae ✅
B. Bacillus cereus
C. Clostridium tetani
D. Listeria monocytogenes
