MOH_Prometric_Microbiologist_20_Scenario_Based_MCQs
                     MOH PROMETRIC EXAM
MICROBIOLOGIST — 20 SCENARIO-BASED MCQs
Practice questions with answers and concise explanations
1. Scenario
A 28-year-old man presents with fever, productive cough, night sweats, and weight loss. Chest imaging
shows an upper-lobe cavitary lesion. Three sputum specimens are sent for testing. Which laboratory method
is most appropriate for rapid detection of Mycobacterium tuberculosis complex?
A. Gram stain
B. Nucleic acid amplification test
C. India ink preparation
D. KOH mount
Correct Answer: B
Explanation: A nucleic acid amplification test can rapidly detect M. tuberculosis complex directly from respiratory
specimens. Acid-fast staining and culture remain important for additional evaluation, including susceptibility testing.
2. Scenario
A hospitalized patient develops fever and hypotension. Blood cultures from two separate sets grow
gram-positive cocci in clusters that are catalase-positive and coagulase-positive. Which organism is most
likely?
A. Staphylococcus epidermidis
B. Staphylococcus aureus
C. Streptococcus pyogenes
D. Enterococcus faecalis
Correct Answer: B
Explanation: S. aureus is a gram-positive coccus in clusters that is catalase-positive and coagulase-positive. Coagulase
helps distinguish it from most coagulase-negative staphylococci.
3. Scenario
A 19-year-old university student presents with fever, severe headache, neck stiffness, and photophobia. CSF
shows neutrophilic leukocytosis, low glucose, and elevated protein. Gram stain demonstrates gram-negative
diplococci. Which organism is most likely?
A. Neisseria meningitidis
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Listeria monocytogenes
Correct Answer: A
Explanation: Neisseria meningitidis typically appears as gram-negative diplococci and can cause acute bacterial
meningitis with neutrophilic CSF, low glucose, and elevated protein.
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4. Scenario
A 65-year-old patient develops watery diarrhea after receiving broad-spectrum antibiotics. Stool testing
detects toxins produced by an anaerobic, spore-forming gram-positive bacillus. Which pathogen is most
likely?
A. Clostridioides difficile
B. Clostridium perfringens
C. Bacillus cereus
D. Staphylococcus aureus
Correct Answer: A
Explanation: C. difficile is associated with antibiotic-associated diarrhea and colitis. Detection of its toxins or toxigenic
organism supports the diagnosis in an appropriate clinical setting.
5. Scenario
A urine culture from a patient with dysuria grows lactose-fermenting gram-negative rods. The isolate is
indole-positive and produces a characteristic metallic sheen on some differential media. Which organism is
most likely?
A. Klebsiella pneumoniae
B. Proteus mirabilis
C. Escherichia coli
D. Pseudomonas aeruginosa
Correct Answer: C
Explanation: E. coli is a common urinary pathogen and is typically a lactose-fermenting, indole-positive gram-negative
rod. Its characteristic appearance on differential media can support identification.
6. Scenario
A burn patient develops wound infection. The culture grows a non-lactose-fermenting gram-negative rod with
a blue-green pigment and a fruity odor. Which organism is most likely?
A. Acinetobacter baumannii
B. Pseudomonas aeruginosa
C. Salmonella enterica
D. Shigella sonnei
Correct Answer: B
Explanation: P. aeruginosa is a non-lactose-fermenting gram-negative rod that may produce blue-green pigments and a
characteristic fruity odor. It is an important opportunistic pathogen in burn wounds.
7. Scenario
A patient has fever and a urinary tract infection. The urine isolate is a gram-negative rod that produces a
spreading growth pattern on agar and is urease-positive. Which organism is most likely?
A. Proteus mirabilis
B. Escherichia coli
C. Enterobacter cloacae
D. Serratia marcescens
Correct Answer: A
Explanation: Proteus species are strongly urease-positive and may show characteristic swarming motility on solid media.
P. mirabilis is a common urinary pathogen.
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8. Scenario
A young adult develops severe diarrhea after eating undercooked poultry. Stool culture yields a curved,
gram-negative, oxidase-positive organism that grows well at a temperature above that used for many routine
enteric bacteria. Which pathogen is most likely?
A. Campylobacter jejuni
B. Shigella dysenteriae
C. Vibrio cholerae
D. Yersinia enterocolitica
Correct Answer: A
Explanation: Campylobacter jejuni is a curved or gull-wing-shaped gram-negative rod associated with poultry and
foodborne gastroenteritis. It is microaerophilic and commonly recovered under selective conditions.
9. Scenario
A patient presents with profuse watery diarrhea and severe dehydration after exposure to contaminated
water. Stool culture suggests a comma-shaped, oxidase-positive gram-negative organism. Which pathogen is
most likely?
A. Vibrio cholerae
B. Campylobacter jejuni
C. Aeromonas hydrophila
D. Salmonella Typhi
Correct Answer: A
Explanation: Vibrio cholerae causes acute watery diarrhea that can lead to profound dehydration. The organism is a
curved gram-negative rod and is oxidase-positive.
10. Scenario
A 42-year-old patient has fever and abdominal pain after eating inadequately cooked eggs. Blood culture
grows a motile gram-negative rod that produces hydrogen sulfide. Which organism is most likely?
A. Salmonella enterica
B. Shigella flexneri
C. Yersinia pestis
D. Klebsiella pneumoniae
Correct Answer: A
Explanation: Salmonella species are motile gram-negative rods and many clinically important strains produce hydrogen
sulfide. Exposure to undercooked eggs is a recognized risk factor for salmonellosis.
11. Scenario
A woman develops dysuria and vaginal irritation. Microscopy of a vaginal specimen shows budding yeast
cells with pseudohyphae. Which organism is most likely?
A. Candida albicans
B. Cryptococcus neoformans
C. Aspergillus fumigatus
D. Malassezia furfur
Correct Answer: A
Explanation: Candida albicans commonly appears as budding yeast with pseudohyphae. It is a frequent cause of
vulvovaginal candidiasis and can also cause invasive infection in susceptible patients.
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12. Scenario
A patient with advanced immunosuppression presents with headache and signs of meningitis. CSF
examination shows encapsulated yeast. Which test is particularly useful for rapid detection of Cryptococcus
antigen?
A. Cryptococcal antigen test
B. Coagulase test
C. Oxidase test
D. Urease breath test
Correct Answer: A
Explanation: Cryptococcal antigen testing is a sensitive method for detecting Cryptococcus species in appropriate clinical
specimens, especially CSF in suspected cryptococcal meningitis.
13. Scenario
A patient with neutropenia develops fever and pulmonary nodules. A respiratory specimen demonstrates
septate hyphae with acute-angle branching. Which fungus is most likely?
A. Aspergillus species
B. Candida species
C. Mucor species
D. Cryptococcus species
Correct Answer: A
Explanation: Aspergillus commonly forms septate hyphae with acute-angle branching. Invasive pulmonary aspergillosis is
an important concern in severely immunocompromised or neutropenic patients.
14. Scenario
A patient develops rapidly progressive sinus and orbital infection in the setting of severe uncontrolled
diabetes. Tissue shows broad, ribbon-like, pauciseptate hyphae with irregular branching. Which group is most
likely?
A. Mucorales
B. Aspergillus
C. Candida
D. Dermatophytes
Correct Answer: A
Explanation: Mucorales can cause rapidly progressive rhino-orbital disease, particularly in patients with severe metabolic
abnormalities such as uncontrolled diabetes. The hyphae are typically broad and pauciseptate.
15. Scenario
A throat swab from a child with pharyngitis grows beta-hemolytic colonies. The organism is a gram-positive
coccus in chains and is PYR-positive. Which organism is most likely?
A. Streptococcus pyogenes
B. Streptococcus pneumoniae
C. Enterococcus faecium
D. Staphylococcus aureus
Correct Answer: A
Explanation: S. pyogenes is a group A beta-hemolytic streptococcus and is typically PYR-positive. It is a major cause of
bacterial pharyngitis.
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16. Scenario
A patient with endocarditis has blood cultures growing gram-positive cocci in pairs and short chains. The
isolate is alpha-hemolytic and optochin-sensitive. Which organism is most likely?
A. Streptococcus pneumoniae
B. Viridans streptococci
C. Enterococcus faecalis
D. Streptococcus pyogenes
Correct Answer: A
Explanation: S. pneumoniae is an alpha-hemolytic, gram-positive diplococcus that is classically optochin-sensitive.
Viridans streptococci are generally optochin-resistant.
17. Scenario
A blood culture grows a gram-positive coccus that is catalase-negative, bile-esculin-positive, and grows in the
presence of 6.5% sodium chloride. Which organism is most likely?
A. Enterococcus species
B. Staphylococcus aureus
C. Streptococcus pyogenes
D. Corynebacterium diphtheriae
Correct Answer: A
Explanation: Enterococci are catalase-negative gram-positive cocci and characteristically can grow in 6.5% NaCl and
hydrolyze esculin in the presence of bile.
18. Scenario
A patient with suspected diphtheria has a throat swab sent to the laboratory. Which finding would support
identification of Corynebacterium diphtheriae?
A. Gram-positive pleomorphic rods with metachromatic granules
B. Gram-negative diplococci
C. Acid-fast branching rods
D. Encapsulated budding yeast
Correct Answer: A
Explanation: C. diphtheriae is a pleomorphic gram-positive bacillus. Metachromatic granules may be seen with
appropriate staining methods. Toxin testing is important when toxigenic diphtheria is suspected.
19. Scenario
A patient with fever and a history of animal exposure develops an ulcer with a black eschar. The laboratory
receives a suspicious specimen containing large gram-positive rods. Which organism should be considered?
A. Bacillus anthracis
B. Bacillus cereus
C. Clostridium tetani
D. Listeria monocytogenes
Correct Answer: A
Explanation: Bacillus anthracis is a large gram-positive, spore-forming bacillus associated with cutaneous anthrax, which
can present with a painless ulcer and characteristic black eschar. Suspected isolates require appropriate laboratory
biosafety procedures.
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20. Scenario
A hospitalized patient has repeated blood cultures positive for a gram-positive, catalase-positive coccus in
clusters. The isolate is coagulase-negative and is recovered from a patient with an indwelling central venous
catheter. Which organism is most likely?
A. Staphylococcus epidermidis
B. Staphylococcus aureus
C. Streptococcus agalactiae
D. Enterococcus faecalis
Correct Answer: A
Explanation: Coagulase-negative staphylococci, particularly S. epidermidis, are associated with infections involving
prosthetic material and intravascular devices because of their ability to form biofilms.
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