Medical Laboratory Technician Exam Medical Laboratory Technician Clinical Microbiology 1 — Questions and Answers
Question 1: A wound culture from a diabetic foot ulcer grows a Gram-positive coccus in clusters that tests positive for coagulase. The isolate is resistant to oxacillin. What is the most appropriate identification?
- Staphylococcus epidermidis
- Methicillin-resistant Staphylococcus aureus (MRSA) (Correct answer)
- Streptococcus pyogenes
- Enterococcus faecalis
Correct answer: Methicillin-resistant Staphylococcus aureus (MRSA)
Coagulase-positive Gram-positive cocci in clusters are Staphylococcus aureus. Oxacillin resistance defines MRSA, which is clinically significant and requires contact precautions and specific antibiotic management.
Question 2: A CSF specimen from a patient with meningitis is sent for fungal culture. After 5 days at 30°C, a mucoid, cream-colored colony appears. India ink preparation reveals encapsulated yeast cells. What is the most likely organism?
- Candida albicans
- Aspergillus fumigatus
- Cryptococcus neoformans (Correct answer)
- Histoplasma capsulatum
Correct answer: Cryptococcus neoformans
Cryptococcus neoformans produces a characteristic polysaccharide capsule visible as a clear halo around yeast cells with India ink stain. It is the leading cause of fungal meningitis, especially in immunocompromised patients.
Question 3: A stool culture is plated on Hektoen enteric (HE) agar. After 24 hours, blue-green colonies with black centers are observed. Which organism is most likely responsible?
- Escherichia coli
- Salmonella typhimurium (Correct answer)
- Shigella sonnei
- Klebsiella pneumoniae
Correct answer: Salmonella typhimurium
Salmonella species produce blue-green colonies with black centers on HE agar due to hydrogen sulfide (H2S) production. Shigella produces green colonies without black centers; E. coli and Klebsiella are inhibited or produce salmon-colored colonies.
Question 4: A sputum culture from an immunocompromised patient grows a mold with septate hyphae that branch at 45-degree angles. Conidiophores bear chains of conidia from a single point. What is the most likely identification?
- Rhizopus species
- Aspergillus fumigatus (Correct answer)
- Fusarium species
- Mucor species
Correct answer: Aspergillus fumigatus
Aspergillus fumigatus produces septate hyphae with characteristic 45-degree branching. Its conidiophores terminate in a flask-shaped vesicle bearing phialides that produce chains of conidia. Rhizopus and Mucor have non-septate (coenocytic) hyphae.
Question 5: A throat swab from a patient with pharyngitis is plated on sheep blood agar. After 24 hours, a beta-hemolytic colony is observed. The isolate is catalase-negative, PYR-positive, and bacitracin-susceptible. What is the most likely organism?
- Streptococcus pneumoniae
- Streptococcus agalactiae (Group B)
- Streptococcus pyogenes (Group A) (Correct answer)
- Enterococcus faecalis
Correct answer: Streptococcus pyogenes (Group A)
Streptococcus pyogenes (Group A Strep) is characteristically beta-hemolytic, catalase-negative, PYR-positive, and susceptible to bacitracin (A disc). Group B Strep is bacitracin-resistant, and Enterococcus is PYR-positive but not beta-hemolytic on sheep blood agar.
Question 6: A technician receives a vaginal swab and must culture for Neisseria gonorrhoeae. Which combination of media and incubation conditions is most appropriate?
- MacConkey agar at 37°C in ambient air
- Thayer-Martin agar at 35-37°C in 5% CO2 (Correct answer)
- Columbia CNA agar at 37°C anaerobically
- Chocolate agar at 42°C in microaerophilic conditions
Correct answer: Thayer-Martin agar at 35-37°C in 5% CO2
Neisseria gonorrhoeae requires enriched, selective media. Thayer-Martin agar contains vancomycin, colistin, and nystatin to suppress normal flora while supporting gonococcal growth. Incubation at 35-37°C with 5% CO2 (capnophilic conditions) is essential for primary isolation.
Medical Laboratory Technician Exam Medical Laboratory Technician Clinical Microbiology 1
A wound culture from a diabetic foot ulcer grows a Gram-positive coccus in clusters that tests positive for coagulase.
The isolate is resistant to oxacillin.
What is the most appropriate identification?