Clinical microbiology: carbapenemase 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.

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Microbiology · Clinical microbiology: carbapenemase detection

Sample item

An ICU isolate of Klebsiella pneumoniae is resistant to meropenem. In a combined disc test, the imipenem disc with EDTA gives a clearly larger zone than the imipenem disc alone. What type of enzyme does this indicate?

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

The answer and the reasoning

D. A metallo-beta-lactamase such as NDM-1, which needs zinc

EDTA chelates zinc, which metallo-beta-lactamases such as NDM-1, VIM and IMP need at the active site, so the imipenem zone is restored. NDM-1 is a major carbapenemase in Indian Enterobacterales, and these enzymes are not inhibited by clavulanate or avibactam, so treatment options include ceftazidime-avibactam with aztreonam or colistin-based regimens.

Option by option

  • A. An AmpC cephalosporinase inhibited by boronic acid

    AmpC enzymes are inhibited by cloxacillin or boronic acid and do not need zinc.

  • B. A KPC serine carbapenemase inhibited by boronic acid

    KPC enzymes are serine carbapenemases detected by boronic acid tests, not by EDTA.

  • C. A serine extended-spectrum beta-lactamase inhibited by clavulanate

    A serine ESBL is blocked by clavulanate, not EDTA, and does not hydrolyse carbapenems appreciably.

  • D. A metallo-beta-lactamase such as NDM-1, which needs zinc (correct)

    Correct. Zone enhancement with EDTA shows a zinc-dependent metallo-enzyme.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, A metallo-beta-lactamase such as NDM-1, which needs zinc. EDTA chelates zinc, which metallo-beta-lactamases such as NDM-1, VIM and IMP need at the active site, so the imipenem zone is restored.

Why is "An AmpC cephalosporinase inhibited by boronic acid" not the answer?

AmpC enzymes are inhibited by cloxacillin or boronic acid and do not need zinc.

Why is "A KPC serine carbapenemase inhibited by boronic acid" not the answer?

KPC enzymes are serine carbapenemases detected by boronic acid tests, not by EDTA.

Why is "A serine extended-spectrum beta-lactamase inhibited by clavulanate" not the answer?

A serine ESBL is blocked by clavulanate, not EDTA, and does not hydrolyse carbapenems appreciably.

What concept does this question test?

It tests Clinical microbiology: carbapenemase detection in Microbiology. NDM-1 is a major carbapenemase in Indian Enterobacterales, and these enzymes are not inhibited by clavulanate or avibactam, so treatment options include ceftazidime-avibactam with aztreonam or colistin-based regimens.

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: carbapenemase 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.