Medical Microbiology Prometric exam questions with answers

1. Question 1Antimicrobial Susceptibility Testing and Resistancemedium

   Which agent is recommended as the disk diffusion surrogate for detecting mecA-mediated oxacillin resistance in Staphylococcus aureus?

   1. AVancomycin
   2. BPenicillin
   3. CCefoxitin
   4. DCeftaroline
   Show answer and explanation▾

   Answer: C. Cefoxitin

   Cefoxitin is a strong inducer of mecA and is the preferred disk surrogate for detecting methicillin resistance. Vancomycin must not be tested by disk diffusion in S. aureus, and ceftaroline is tested for its own clinical use.

2. Question 2Antimicrobial Susceptibility Testing and Resistancehard

   Clinicians request colistin susceptibility on a carbapenem-resistant Acinetobacter baumannii. Which method is recommended for reliable colistin MIC determination?

   1. AAutomated system result alone
   2. BDisk diffusion using a 10 microgram disk
   3. CGradient diffusion strip on Mueller-Hinton agar
   4. DBroth microdilution without polysorbate 80
   Show answer and explanation▾

   Answer: D. Broth microdilution without polysorbate 80

   The joint CLSI-EUCAST recommendation is broth microdilution in untreated plates without surfactants. Polymyxins diffuse poorly in agar, so disk and gradient methods give unreliable results.

3. Question 3Clinical Bacteriologymedium

   A neonate with meningitis has small beta-haemolytic Gram-positive rods in CSF culture. The isolate is catalase positive and shows tumbling motility at room temperature. What is the most likely organism?

   1. ACorynebacterium striatum
   2. BStreptococcus agalactiae
   3. CListeria monocytogenes
   4. DBacillus cereus
   Show answer and explanation▾

   Answer: C. Listeria monocytogenes

   Listeria is a catalase-positive, beta-haemolytic Gram-positive rod with tumbling motility at 20 to 25 degrees Celsius. Group B streptococcus is a catalase-negative coccus, which is the main cause of confusion when Listeria appears coccoid.

4. Question 4Infection Prevention, Control and Epidemiologymedium

   Five MRSA bloodstream isolates appear in an ICU within 3 weeks. Which method gives the highest discrimination to determine whether they represent a single transmission cluster?

   1. AAntibiogram comparison
   2. BWhole genome sequencing
   3. CPhage typing
   4. DBiochemical profiling
   Show answer and explanation▾

   Answer: B. Whole genome sequencing

   Whole genome sequencing gives single-nucleotide resolution and is the most discriminatory typing method for outbreak investigation. Antibiograms and biochemical profiles are too crude to establish relatedness.

5. Question 5Laboratory Methods and Specimen Managementeasy

   A trainee's Gram stains show Staphylococcus aureus control organisms appearing pink. Which step of the procedure was most likely performed incorrectly?

   1. AExcessive heat fixation of the smear
   2. BOmission of the safranin counterstain
   3. CProlonged crystal violet staining
   4. DExcessive decolorisation with acetone-alcohol
   Show answer and explanation▾

   Answer: D. Excessive decolorisation with acetone-alcohol

   Decolorisation is the critical step; over-decolorising removes the crystal violet-iodine complex from Gram-positive cells so they take up safranin and appear pink. Omitting safranin would leave Gram-negative cells colourless rather than turning Gram-positives pink.

6. Question 6Laboratory Methods and Specimen Managementmedium

   An audit finds a high rate of falsely negative blood cultures in adults. Which single collection factor has the greatest effect on blood culture yield?

   1. AThe interval between drawing each set
   2. BThe volume of blood inoculated per bottle
   3. CWhether blood is drawn from an arm or a leg
   4. DThe temperature during transport
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   Answer: B. The volume of blood inoculated per bottle

   Yield rises directly with the volume of blood cultured, so adult bottles should receive about 8 to 10 mL each, usually as two or more sets. Timing between sets matters far less than adequate volume.

7. Question 7Virology and Molecular Diagnosticshard

   A fourth-generation HIV-1/2 antigen-antibody immunoassay is repeatedly reactive. The HIV-1/HIV-2 antibody differentiation assay is negative. What is the most appropriate next test?

   1. ACD4 lymphocyte count
   2. BHIV-1 Western blot
   3. CRepeat the immunoassay in 6 months
   4. DHIV-1 nucleic acid test
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   Answer: D. HIV-1 nucleic acid test

   A reactive combination assay with a negative differentiation assay may represent acute infection before antibodies develop, so HIV-1 RNA testing is required. The Western blot is no longer recommended and would also miss early infection.

8. Question 8Virology and Molecular Diagnosticseasy

   Two patients have positive real-time PCR results for the same respiratory virus with cycle threshold values of 18 and 34. What does the difference mean?

   1. AThe sample with Ct 18 has more target nucleic acid
   2. BThe sample with Ct 34 has more target nucleic acid
   3. CBoth samples contain the same viral load
   4. DThe sample with Ct 18 is a false positive
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   Answer: A. The sample with Ct 18 has more target nucleic acid

   The cycle threshold is the number of cycles needed for signal to cross the threshold, so lower values mean more starting target. A Ct of 34 indicates much less target nucleic acid.

9. Question 9Laboratory Methods and Specimen Managementmedium

   A stool from a patient with profuse rice-water diarrhoea is cultured on thiosulfate citrate bile salts sucrose agar. What colony appearance is expected for Vibrio cholerae?

   1. AYellow colonies from sucrose fermentation
   2. BGreen colonies from absence of sucrose use
   3. CBlack-centred colonies from hydrogen sulfide
   4. DPink colonies from lactose fermentation
   Show answer and explanation▾

   Answer: A. Yellow colonies from sucrose fermentation

   V. cholerae ferments sucrose and produces yellow colonies on TCBS. V. parahaemolyticus does not ferment sucrose and gives green colonies, and pink colonies describe lactose fermenters on MacConkey agar.

10. Question 10Medical Mycologymedium

    An HIV-positive man with a CD4 count of 40 cells per microlitre has headache and raised CSF opening pressure. Which test on CSF is most sensitive for rapid diagnosis of the likely infection?

    1. AIndia ink microscopy
    2. BCryptococcal antigen test
    3. CGram stain microscopy
    4. DFungal culture on agar
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    Answer: B. Cryptococcal antigen test

    Cryptococcal antigen testing is highly sensitive and rapid and is the preferred diagnostic test. India ink is quick but misses a substantial proportion of cases, and culture takes several days.

11. Question 11Mycobacteriologymedium

    A smear-positive sputum is tested by Xpert MTB/RIF and shows M. tuberculosis detected, rifampicin resistance detected. Mutations in which gene are being detected?

    1. AkatG
    2. BrpoB
    3. CinhA
    4. DembB
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    Answer: B. rpoB

    Xpert MTB/RIF detects mutations in the 81-base-pair rifampicin resistance-determining region of rpoB. katG and inhA mutations confer isoniazid resistance, and embB relates to ethambutol.

12. Question 12Mycobacteriologymedium

    Decontaminated sputum is inoculated into an automated liquid mycobacterial culture system. For how long should a tube be incubated before it is reported as negative?

    1. A12 weeks
    2. B2 weeks
    3. C3 weeks
    4. D6 weeks
    Show answer and explanation▾

    Answer: D. 6 weeks

    Automated liquid culture systems such as MGIT use a standard 42-day (6-week) protocol before a negative result is reported. Shorter incubation would miss slow-growing isolates, while solid media are traditionally held for 8 weeks.

13. Question 13Mycobacteriologymedium

    A healthcare worker has a positive interferon-gamma release assay on pre-employment screening. What does this result alone indicate?

    1. ATuberculosis infection, latent or active
    2. BActive pulmonary tuberculosis
    3. CPrior BCG vaccination
    4. DNon-tuberculous mycobacterial infection
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    Answer: A. Tuberculosis infection, latent or active

    An IGRA shows an immune response to M. tuberculosis antigens but cannot distinguish latent infection from active disease, so clinical assessment and chest imaging are needed. Unlike the tuberculin skin test, it is not affected by BCG vaccination.

14. Question 14Parasitologymedium

    A painless ulcerated nodule on the forearm has persisted for 3 months. A slit-skin smear stained with Giemsa shows small oval intracellular bodies with a nucleus and a rod-shaped kinetoplast within macrophages. What are these structures?

    1. AToxoplasma tachyzoites
    2. BHistoplasma yeasts
    3. CLeishmania amastigotes
    4. DTrypanosoma trypomastigotes
    Show answer and explanation▾

    Answer: C. Leishmania amastigotes

    Intracellular amastigotes with both a nucleus and a kinetoplast are diagnostic of leishmaniasis. Histoplasma yeasts are similar in size but lack a kinetoplast, and trypomastigotes are extracellular flagellated forms.

15. Question 15Virology and Molecular Diagnosticsmedium

    A pre-operative screen shows HBsAg negative, total anti-HBc positive and anti-HBs positive. What is the most likely interpretation?

    1. AResolved past hepatitis B infection
    2. BImmunity from hepatitis B vaccination only
    3. CChronic hepatitis B infection
    4. DAcute hepatitis B infection
    Show answer and explanation▾

    Answer: A. Resolved past hepatitis B infection

    Anti-HBc with anti-HBs and negative HBsAg indicates immunity from past natural infection. Vaccine-induced immunity gives anti-HBs alone without anti-HBc.

Last reviewed September 2026. Questions are original and written to the published exam blueprint; they are not taken from any real paper.