Clinical microbiology and hospital infection control: vancomycin testing in S. aureus: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Clinical microbiology and hospital infection control: vancomycin testing in S. aureus

Sample item

A laboratory tests the vancomycin susceptibility of Staphylococcus aureus by disc diffusion only. Why is this method not appropriate?

ADisc zones cannot reliably detect reduced-susceptibility strains, so an MIC method is requiredBStaphylococci do not grow on Mueller-Hinton agarCDisc diffusion cannot detect methicillin resistance in any caseDVancomycin is inactivated on Mueller-Hinton agar

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Disc zones cannot reliably detect reduced-susceptibility strains, so an MIC method is required

Vancomycin diffuses slowly and zone sizes do not separate susceptible from intermediate or resistant strains. Reliable testing for S. aureus needs an MIC method, such as broth microdilution, agar dilution or gradient strips.

Option by option

A. Disc zones cannot reliably detect reduced-susceptibility strains, so an MIC method is required (correct)
Correct. Disc zones do not discriminate reduced vancomycin susceptibility, so MIC testing is used.
B. Staphylococci do not grow on Mueller-Hinton agar
Staphylococci grow well on Mueller-Hinton agar.
C. Disc diffusion cannot detect methicillin resistance in any case
Methicillin resistance is detected by cefoxitin discs, which is a separate issue.
D. Vancomycin is inactivated on Mueller-Hinton agar
Vancomycin is not inactivated by the agar medium.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Disc zones cannot reliably detect reduced-susceptibility strains, so an MIC method is required. Vancomycin diffuses slowly and zone sizes do not separate susceptible from intermediate or resistant strains.

Why is "Staphylococci do not grow on Mueller-Hinton agar" not the answer?

Staphylococci grow well on Mueller-Hinton agar.

Why is "Disc diffusion cannot detect methicillin resistance in any case" not the answer?

Methicillin resistance is detected by cefoxitin discs, which is a separate issue.

Why is "Vancomycin is inactivated on Mueller-Hinton agar" not the answer?

Vancomycin is not inactivated by the agar medium.

What concept does this question test?

It tests Clinical microbiology and hospital infection control: vancomycin testing in S. aureus in Microbiology. Reliable testing for S. aureus needs an MIC method, such as broth microdilution, agar dilution or gradient strips.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Clinical microbiology and hospital infection control: vancomycin testing in S. aureus?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.