Microbiology Laboratory Prometric exam questions with answers

1. Question 1Antimicrobial Susceptibility Testing and Resistancemedium

   Which disc is recommended as the phenotypic surrogate to detect mecA-mediated methicillin resistance in Staphylococcus aureus by disc diffusion?

   1. APenicillin
   2. BVancomycin
   3. COxacillin
   4. DCefoxitin
   Show answer and explanation▾

   Answer: D. Cefoxitin

   Cefoxitin is a strong inducer of mecA expression and gives clearer separation between resistant and susceptible strains by disc diffusion, so it is the recommended surrogate. Oxacillin discs perform poorly for disc diffusion. Penicillin resistance is mainly due to beta-lactamase.

2. Question 2Antimicrobial Susceptibility Testing and Resistancehard

   A Klebsiella pneumoniae isolate has reduced susceptibility to meropenem. Which phenotypic test detects carbapenemase production by incubating a meropenem disc in a suspension of the isolate?

   1. ADouble-disc synergy test with clavulanate
   2. BD-zone test with erythromycin
   3. CModified carbapenem inactivation method
   4. DCefoxitin disc screening test
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   Answer: C. Modified carbapenem inactivation method

   In the modified carbapenem inactivation method (mCIM), a meropenem disc is incubated in the isolate suspension and then placed on a lawn of a susceptible E. coli. If the isolate produces a carbapenemase, the drug is destroyed and the zone is small or absent. Molecular or immunochromatographic tests can then identify the enzyme type.

3. Question 3Bacteriology and Identificationmedium

   A urine culture grows a lactose non-fermenter that spreads in waves across the blood agar plate. It is urease positive and produces H2S. What is the most likely organism?

   1. AEscherichia coli
   2. BProteus mirabilis
   3. CSalmonella enterica
   4. DPseudomonas aeruginosa
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   Answer: B. Proteus mirabilis

   Swarming motility on blood agar, strong urease and H2S production are typical of Proteus mirabilis. Urease production promotes struvite stone formation. Salmonella produces H2S but is urease negative and does not swarm.

4. Question 4Biosafety, Infection Prevention and Qualitymedium

   The daily disc diffusion quality control with E. coli ATCC 25922 gives a gentamicin zone outside the acceptable range. What should be done with today's patient gentamicin results?

   1. AReport them, as patient strains differ
   2. BReport them with a disclaimer comment
   3. CAdjust the zones by the size of the error
   4. DWithhold them and investigate the cause
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   Answer: D. Withhold them and investigate the cause

   An out-of-range control result means the test system may be faulty, so patient results for that drug should not be reported until the cause, such as disc potency, medium or inoculum, is found and corrected. Adjusting zones or adding disclaimers is not acceptable practice. Corrective action must be documented.

5. Question 5Specimen Collection, Transport and Processingeasy

   A nurse asks how much blood to put into each adult blood culture bottle for the best detection of bacteraemia. What is the recommended volume per bottle?

   1. A1 to 2 mL
   2. B8 to 10 mL
   3. C3 to 5 mL
   4. D20 to 25 mL
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   Answer: B. 8 to 10 mL

   Most adult blood culture bottles are designed for 8 to 10 mL, and detection rises with the volume of blood cultured. Underfilled bottles are a common cause of missed bacteraemia. Overfilling can increase false positives in some systems.

6. Question 6Molecular Diagnosticsmedium

   Two samples are tested by real-time PCR for the same target. Sample A has a Ct of 18 and sample B has a Ct of 32. What does this indicate?

   1. ASample A has more target than sample B
   2. BSample B has more target than sample A
   3. CBoth samples have equal amounts of target
   4. DSample A is likely a false positive
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   Answer: A. Sample A has more target than sample B

   The Ct value is the cycle at which fluorescence crosses the threshold, so a lower Ct means more starting target. With efficient amplification each cycle roughly doubles the product, so 14 cycles represent a very large difference in quantity. A high Ct can reflect a low load or contamination and needs clinical interpretation.

7. Question 7Parasitologyhard

   A stool from a child with watery diarrhoea is stained with a modified Ziehl-Neelsen stain. Round, pink-red oocysts of about 4 to 6 micrometres are seen. What is the most likely organism?

   1. ACyclospora cayetanensis
   2. BCryptosporidium species
   3. CCystoisospora belli
   4. DGiardia duodenalis
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   Answer: B. Cryptosporidium species

   Cryptosporidium oocysts are about 4 to 6 micrometres and stain pink-red with modified acid-fast stains. Cyclospora oocysts are larger, about 8 to 10 micrometres, and stain variably. Cystoisospora oocysts are much larger and oval, while Giardia is not acid fast.

8. Question 8Virology and Serologyeasy

   Which sample is generally preferred for PCR detection of influenza in a patient with acute respiratory symptoms?

   1. ANasopharyngeal swab in transport medium
   2. BMidstream urine in a sterile pot
   3. CSerum for antibody testing
   4. DStool in a sterile container
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   Answer: A. Nasopharyngeal swab in transport medium

   A nasopharyngeal swab placed in viral transport medium gives good viral yield for PCR in acute respiratory infection. Serology is retrospective and not useful for acute diagnosis. Urine and stool are not appropriate samples for influenza.

9. Question 9Molecular Diagnosticsmedium

   MALDI-TOF mass spectrometry is used to identify a bacterial colony. What is mainly measured to produce the identification?

   1. ADNA sequence of the 16S rRNA gene
   2. BCarbohydrate fermentation patterns
   3. CFatty acid profile by gas chromatography
   4. DMass spectrum of mainly ribosomal proteins
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   Answer: D. Mass spectrum of mainly ribosomal proteins

   MALDI-TOF ionises proteins from the colony, mostly abundant ribosomal proteins, and compares the resulting mass spectrum with a reference database. 16S sequencing and fatty acid analysis are different methods. Identification depends on the quality of the database.

10. Question 10Mycologymedium

    A patient with advanced HIV has headache and fever. Which test on CSF is most sensitive for cryptococcal meningitis?

    1. AIndia ink stain
    2. BGram stain
    3. CCalcofluor white stain
    4. DCryptococcal antigen test
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    Answer: D. Cryptococcal antigen test

    Cryptococcal antigen detection, for example by lateral flow assay, is more sensitive than India ink microscopy and is the preferred rapid test. India ink shows encapsulated yeasts but misses low-burden infection. Culture should also be performed.

11. Question 11Parasitologymedium

    A patient with chronic diarrhoea needs stool microscopy for ova, cysts and parasites. Why are three samples on separate days often requested?

    1. AParasites only appear in the evening
    2. BOne sample is used for each stain
    3. CShedding of cysts and eggs is intermittent
    4. DThree samples are needed for culture
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    Answer: C. Shedding of cysts and eggs is intermittent

    Many intestinal parasites shed cysts and eggs intermittently, so a single negative sample does not exclude infection. Examining multiple samples on different days increases sensitivity. Antigen or molecular tests can reduce the need for repeat samples for some organisms.

12. Question 12Specimen Collection, Transport and Processingmedium

    A midstream urine sample without preservative cannot reach the laboratory for 6 hours. What should be done to keep the culture result valid?

    1. AKeep it at room temperature
    2. BRefrigerate it at about 4 degrees Celsius
    3. CIncubate it at 37 degrees Celsius
    4. DFreeze it at -20 degrees Celsius
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    Answer: B. Refrigerate it at about 4 degrees Celsius

    Bacteria multiply in urine at room temperature, so unpreserved urine should be cultured within about 2 hours or refrigerated, which preserves counts for up to 24 hours. Incubation would falsely raise the count. Freezing can damage organisms and cells.

13. Question 13Specimen Collection, Transport and Processingmedium

    A Gram stain of an expectorated sputum shows more than 25 squamous epithelial cells and 5 neutrophils per low power field. What should the laboratory do?

    1. AReject it as saliva and request a repeat
    2. BCulture it and report all organisms
    3. CCulture it for anaerobes only
    4. DReport it as a normal sputum
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    Answer: A. Reject it as saliva and request a repeat

    Many squamous epithelial cells with few neutrophils show heavy oral contamination, so culture results would mainly reflect mouth flora. Screening criteria such as fewer than 10 squamous cells per low power field are used to accept samples. Rejecting and requesting a repeat avoids misleading reports.

14. Question 14Virology and Serologymedium

    A patient's hepatitis B results are: HBsAg negative, anti-HBc total positive and anti-HBs positive. What is the most likely interpretation?

    1. AImmunity from vaccination only
    2. BAcute hepatitis B infection
    3. CChronic hepatitis B infection
    4. DPast infection that has resolved
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    Answer: D. Past infection that has resolved

    Anti-HBc shows past exposure to the virus, and anti-HBs with negative HBsAg shows recovery and immunity. Vaccination produces anti-HBs only, without anti-HBc. Acute or chronic infection would have a positive HBsAg.

15. Question 15Virology and Serologymedium

    Why do fourth-generation HIV screening assays detect infection earlier than third-generation antibody-only assays?

    1. AThey detect p24 antigen as well as antibody
    2. BThey detect HIV RNA directly
    3. CThey detect only IgG antibody
    4. DThey use saliva instead of blood
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    Answer: A. They detect p24 antigen as well as antibody

    Fourth-generation assays detect both HIV antibody and p24 antigen, which appears before antibody, shortening the window period. They do not detect RNA, which requires nucleic acid testing. Reactive results must be confirmed by a supplementary test.

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