SCI 251
Microbiology
Final Exam Review
(With Solutions)
2025
1. A patient presents with signs of bacterial meningitis.
Cerebrospinal fluid (CSF) is collected by lumbar puncture. Which
culture medium provides optimal isolation of Neisseria meningitidis
while suppressing contaminants?
a. MacConkey agar
b. Chocolate agar
c. Thayer–Martin agar
d. Mannitol salt agar
ANS: c. Thayer–Martin agar
Rationale: Thayer–Martin agar contains antibiotics (vancomycin,
colistin, nystatin) that inhibit most bacteria and fungi, allowing
selective growth of Neisseria species.
2. A critically ill patient with suspected Pneumocystis jirovecii
pneumonia undergoes bronchoalveolar lavage. Which stain is most
sensitive for detecting this fungus in respiratory specimens?
a. Gram stain
b. Acid-fast bacilli stain
c. Gomori methenamine silver stain
d. India ink
ANS: c. Gomori methenamine silver stain
Rationale: Silver-stain methods highlight the cyst walls of
Pneumocystis jirovecii, providing high sensitivity in respiratory
samples.
3. A lab implements MALDI-TOF mass spectrometry for bacterial
identification. Which principle underlies accurate species
determination?
a. Chromatographic separation of metabolites
, b. Electrophoretic mobility of DNA fragments
c. Time-of-flight of ionized ribosomal proteins
d. Fluorescent tagging of surface antigens
ANS: c. Time-of-flight of ionized ribosomal proteins
Rationale: MALDI-TOF ionizes proteins, and their flight time
through a vacuum tube generates a mass spectrum unique to each
species.
4. An endoscope tray fails biological spore testing after autoclaving.
Which parameter is most likely insufficient?
a. Chamber vacuum pressure
b. Exposure temperature and duration
c. pH of water used
d. Concentration of sterilant gas
ANS: b. Exposure temperature and duration
Rationale: Steam sterilization requires specific temperature (≥121
°C) held for a minimum time (e.g., 15 minutes) to inactivate spores;
inadequate time or temperature leads to failure.
5. For rapid diagnosis of Clostridioides difficile infection, which
laboratory method offers the best combination of sensitivity and
turnaround time?
a. Toxin A/B enzyme immunoassay (EIA) alone
b. Anaerobic stool culture
c. Nucleic acid amplification test (NAAT) for toxin genes
d. Glutamate dehydrogenase screen only
ANS: c. Nucleic acid amplification test (NAAT) for toxin genes
Rationale: NAATs detect toxin-encoding genes with higher
sensitivity and faster results than culture or EIA alone.
6. Two sets of blood cultures are drawn from a febrile patient. Only
one bottle grows coagulase-negative staphylococci after 72 hours.
Which interpretation is most likely?
a. True bacteremia requiring prolonged antibiotics
b. Laboratory contamination of one culture set
c. Endocarditis by coagulase-negative staphylococci
d. Polymicrobial sepsis
ANS: b. Laboratory contamination of one culture set
Microbiology
Final Exam Review
(With Solutions)
2025
1. A patient presents with signs of bacterial meningitis.
Cerebrospinal fluid (CSF) is collected by lumbar puncture. Which
culture medium provides optimal isolation of Neisseria meningitidis
while suppressing contaminants?
a. MacConkey agar
b. Chocolate agar
c. Thayer–Martin agar
d. Mannitol salt agar
ANS: c. Thayer–Martin agar
Rationale: Thayer–Martin agar contains antibiotics (vancomycin,
colistin, nystatin) that inhibit most bacteria and fungi, allowing
selective growth of Neisseria species.
2. A critically ill patient with suspected Pneumocystis jirovecii
pneumonia undergoes bronchoalveolar lavage. Which stain is most
sensitive for detecting this fungus in respiratory specimens?
a. Gram stain
b. Acid-fast bacilli stain
c. Gomori methenamine silver stain
d. India ink
ANS: c. Gomori methenamine silver stain
Rationale: Silver-stain methods highlight the cyst walls of
Pneumocystis jirovecii, providing high sensitivity in respiratory
samples.
3. A lab implements MALDI-TOF mass spectrometry for bacterial
identification. Which principle underlies accurate species
determination?
a. Chromatographic separation of metabolites
, b. Electrophoretic mobility of DNA fragments
c. Time-of-flight of ionized ribosomal proteins
d. Fluorescent tagging of surface antigens
ANS: c. Time-of-flight of ionized ribosomal proteins
Rationale: MALDI-TOF ionizes proteins, and their flight time
through a vacuum tube generates a mass spectrum unique to each
species.
4. An endoscope tray fails biological spore testing after autoclaving.
Which parameter is most likely insufficient?
a. Chamber vacuum pressure
b. Exposure temperature and duration
c. pH of water used
d. Concentration of sterilant gas
ANS: b. Exposure temperature and duration
Rationale: Steam sterilization requires specific temperature (≥121
°C) held for a minimum time (e.g., 15 minutes) to inactivate spores;
inadequate time or temperature leads to failure.
5. For rapid diagnosis of Clostridioides difficile infection, which
laboratory method offers the best combination of sensitivity and
turnaround time?
a. Toxin A/B enzyme immunoassay (EIA) alone
b. Anaerobic stool culture
c. Nucleic acid amplification test (NAAT) for toxin genes
d. Glutamate dehydrogenase screen only
ANS: c. Nucleic acid amplification test (NAAT) for toxin genes
Rationale: NAATs detect toxin-encoding genes with higher
sensitivity and faster results than culture or EIA alone.
6. Two sets of blood cultures are drawn from a febrile patient. Only
one bottle grows coagulase-negative staphylococci after 72 hours.
Which interpretation is most likely?
a. True bacteremia requiring prolonged antibiotics
b. Laboratory contamination of one culture set
c. Endocarditis by coagulase-negative staphylococci
d. Polymicrobial sepsis
ANS: b. Laboratory contamination of one culture set
