Clinical microbiology: MRSA detection: 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.

Try it

Microbiology · Clinical microbiology: MRSA detection

Sample item

A Staphylococcus aureus isolate from a wound shows a 30 microgram cefoxitin disk zone of 18 mm on disk diffusion. How should the isolate be reported?

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

The answer and the reasoning

A. Methicillin resistant, with resistance to all beta-lactams except anti-MRSA cephalosporins

For S. aureus, a cefoxitin zone of 21 mm or less indicates mecA-mediated methicillin resistance, because cefoxitin is a better inducer of the mecA gene than oxacillin. Such isolates carry the altered penicillin-binding protein PBP2a and are reported as resistant to all penicillins, cephalosporins and carbapenems, except ceftaroline.

Option by option

  • A. Methicillin resistant, with resistance to all beta-lactams except anti-MRSA cephalosporins (correct)

    Correct. A zone of 18 mm is below the breakpoint, indicating MRSA.

  • B. Methicillin sensitive, with reduced susceptibility only

    There is no category of methicillin sensitivity at this zone size.

  • C. Vancomycin resistant

    Vancomycin resistance is detected by MIC testing and is not indicated by a cefoxitin zone.

  • D. Susceptible to cloxacillin but resistant to penicillin

    An MRSA isolate is resistant to cloxacillin and flucloxacillin as well as to penicillin.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Methicillin resistant, with resistance to all beta-lactams except anti-MRSA cephalosporins. For S.

Why is "Methicillin sensitive, with reduced susceptibility only" not the answer?

There is no category of methicillin sensitivity at this zone size.

Why is "Vancomycin resistant" not the answer?

Vancomycin resistance is detected by MIC testing and is not indicated by a cefoxitin zone.

Why is "Susceptible to cloxacillin but resistant to penicillin" not the answer?

An MRSA isolate is resistant to cloxacillin and flucloxacillin as well as to penicillin.

What concept does this question test?

It tests Clinical microbiology: MRSA detection in Microbiology. aureus, a cefoxitin zone of 21 mm or less indicates mecA-mediated methicillin resistance, because cefoxitin is a better inducer of the mecA gene than oxacillin. Such isolates carry the altered penicillin-binding protein PBP2a and are reported as resistant to all penicillins, cephalosporins and carbapenems, except ceftaroline.

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: MRSA detection?

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.