Antimicrobials: D-test and inducible clindamycin resistance: practice question

A Pharmacology 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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Pharmacology · Antimicrobials: D-test and inducible clindamycin resistance

Sample item

A Staphylococcus aureus isolate from an abscess is resistant to erythromycin and appears sensitive to clindamycin. On disc testing the clindamycin zone is flattened next to the erythromycin disc. How should the result be interpreted?

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

The answer and the reasoning

B. Inducible clindamycin resistance; avoid clindamycin

A D-shaped zone indicates that erythromycin induces the erm gene, which methylates the ribosomal target and gives MLSB resistance. Clindamycin may then fail during treatment, so it is reported as resistant.

Option by option

  • A. Beta-lactamase production, so add clavulanate

    Beta-lactamase is detected by a different test and does not cause a D-shaped zone.

  • B. Inducible clindamycin resistance; avoid clindamycin (correct)

    Correct. The D-test shows erm-mediated inducible clindamycin resistance.

  • C. Heteroresistance to vancomycin in this isolate

    Vancomycin susceptibility is tested with other methods and is not the question here.

  • D. True sensitivity, so clindamycin can safely be used

    The flattened zone means the apparent sensitivity is unreliable, and treatment failure can occur.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is B, Inducible clindamycin resistance; avoid clindamycin. A D-shaped zone indicates that erythromycin induces the erm gene, which methylates the ribosomal target and gives MLSB resistance.

Why is "Beta-lactamase production, so add clavulanate" not the answer?

Beta-lactamase is detected by a different test and does not cause a D-shaped zone.

Why is "Heteroresistance to vancomycin in this isolate" not the answer?

Vancomycin susceptibility is tested with other methods and is not the question here.

Why is "True sensitivity, so clindamycin can safely be used" not the answer?

The flattened zone means the apparent sensitivity is unreliable, and treatment failure can occur.

What concept does this question test?

It tests Antimicrobials: D-test and inducible clindamycin resistance in Pharmacology. Clindamycin may then fail during treatment, so it is reported as resistant.

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 Antimicrobials: D-test and inducible clindamycin resistance?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.