DHA Medical Microbiology exam questions

6 worked Medical Microbiology practice questions

Original exam-style questions from the bank, spread across the blueprint domains. They are not real or recalled exam questions. Try each one, then open the answer for the rationale and reference. None of them appear in the free mock exam, so reading them will not spoil it.

1. Question 1 · Antimicrobial Susceptibility Testing and Resistance

   A meropenem-resistant Enterobacter cloacae isolate is positive by the modified carbapenem inactivation method. In the parallel EDTA-modified test, the meropenem zone is 7 mm larger than in the mCIM. Which enzyme class is most likely present?

   1. ASerine carbapenemase such as KPC
   2. BMetallo-beta-lactamase such as NDM
   3. COXA-48-like carbapenemase
   4. DPlasmid-mediated AmpC
   Show answer and explanation

   Correct answer: B. Metallo-beta-lactamase such as NDM

   EDTA chelates zinc and inhibits metallo-beta-lactamases, so a zone increase of 5 mm or more in the eCIM indicates an MBL such as NDM, VIM or IMP. KPC and OXA-48 are serine enzymes not inhibited by EDTA, and AmpC alone does not give a positive mCIM.

   Reference: CLSI M100 Performance Standards for Antimicrobial Susceptibility Testing

2. Question 2 · Clinical Bacteriology

   A vaginal-rectal screening culture at 36 weeks grows a catalase-negative, beta-haemolytic streptococcus. Which test pair supports Streptococcus agalactiae?

   1. ABile esculin positive and 6.5% NaCl growth
   2. BPYR positive and bacitracin susceptible
   3. COptochin susceptible and bile soluble
   4. DCAMP positive and hippurate positive
   Show answer and explanation

   Correct answer: D. CAMP positive and hippurate positive

   Group B streptococci give a positive CAMP reaction and hydrolyse hippurate. PYR positivity with bacitracin susceptibility identifies group A, and bile esculin with salt tolerance identifies enterococci.

   Reference: Bailey and Scott's Diagnostic Microbiology, 15th ed.

3. Question 3 · Clinical Bacteriology

   A traveller has had fever, headache and relative bradycardia for 5 days. Typhoid fever is suspected. Which test is the most useful initial diagnostic investigation?

   1. ABlood culture
   2. BStool culture
   3. CWidal agglutination test
   4. DUrine culture
   Show answer and explanation

   Correct answer: A. Blood culture

   Blood culture has its highest yield in the first week of enteric fever and allows susceptibility testing. Stool and urine cultures become positive later, and the Widal test has poor sensitivity and specificity.

   Reference: Harrison's Principles of Internal Medicine, 21st ed.

4. Question 4 · Clinical Bacteriology

   A two-step Clostridioides difficile algorithm on an unformed stool gives a positive glutamate dehydrogenase antigen and a negative toxin A/B immunoassay. What is the most appropriate next step?

   1. AReport as C. difficile infection
   2. BReport as negative for C. difficile
   3. CRepeat the toxin immunoassay in 24 hours
   4. DArbitrate with a toxin gene NAAT
   Show answer and explanation

   Correct answer: D. Arbitrate with a toxin gene NAAT

   Discordant GDH-positive, toxin-negative results are resolved by a nucleic acid amplification test for the toxin gene, interpreted with clinical findings. Repeat toxin testing within 7 days is discouraged, and GDH alone cannot separate toxigenic from non-toxigenic strains.

   Reference: IDSA/SHEA Clinical Practice Guidelines for Clostridioides difficile Infection 2017

5. Question 5 · Clinical Bacteriology

   A man with severe pneumonia linked to a hotel cooling tower has a negative Legionella urinary antigen test. Why should lower respiratory culture on buffered charcoal yeast extract agar still be sent?

   1. AUrinary antigen is detectable only after 6 weeks
   2. BThe antigen test mainly detects L. pneumophila serogroup 1
   3. CCulture is faster than the antigen test
   4. DUrinary antigen cross-reacts with pneumococcus
   Show answer and explanation

   Correct answer: B. The antigen test mainly detects L. pneumophila serogroup 1

   Urinary antigen tests detect mainly L. pneumophila serogroup 1, so other serogroups and species are missed. Culture on BCYE detects all Legionella species and allows typing for outbreak investigation.

   Reference: Manual of Clinical Microbiology, 12th ed.

6. Question 6 · Medical Mycology

   A yeast from an ICU patient's blood culture is identified as Candida haemulonii by a biochemical identification system and is fluconazole resistant. What is the most appropriate next step?

   1. AConfirm by MALDI-TOF with an updated database
   2. BReport the identification as C. haemulonii
   3. CRepeat the same biochemical panel
   4. DPerform a germ tube test to confirm
   Show answer and explanation

   Correct answer: A. Confirm by MALDI-TOF with an updated database

   Candida auris is commonly misidentified as C. haemulonii by biochemical systems, so confirmation by MALDI-TOF with an updated library or sequencing is needed, with prompt infection control notification. The germ tube test does not identify C. auris.

   Reference: CDC Identification of Candida auris guidance