Clinical microbiology: D-test: 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: D-test

Sample item

Staphylococcus aureus from an abscess is resistant to erythromycin and appears susceptible to clindamycin. When the two discs are placed 15 to 20 mm apart, the clindamycin zone is flattened on the side facing the erythromycin disc, forming a D shape. What does this indicate?

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

The answer and the reasoning

A. Inducible clindamycin resistance through erm genes, so clindamycin should be avoided

Erythromycin induces expression of erm-encoded ribosomal methylase, which methylates 23S rRNA and confers cross-resistance to macrolides, lincosamides and streptogramin B. The inducible phenotype is not seen with clindamycin alone, so the D-test unmasks it, and clindamycin may fail in vivo through selection of constitutive mutants.

Option by option

  • A. Inducible clindamycin resistance through erm genes, so clindamycin should be avoided (correct)

    Correct. The D-shaped zone shows inducible resistance, and clindamycin is unreliable.

  • B. mecA-mediated methicillin resistance

    Methicillin resistance is assessed by cefoxitin disc testing or mecA detection.

  • C. Synergy between the drugs, so they should be given together

    The flattening reflects antagonism through induction and not synergy.

  • D. Beta-lactamase production

    Beta-lactamase production is shown by penicillin zone edge appearance, and not by a D-test.

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 A, Inducible clindamycin resistance through erm genes, so clindamycin should be avoided. Erythromycin induces expression of erm-encoded ribosomal methylase, which methylates 23S rRNA and confers cross-resistance to macrolides, lincosamides and streptogramin B.

Why is "mecA-mediated methicillin resistance" not the answer?

Methicillin resistance is assessed by cefoxitin disc testing or mecA detection.

Why is "Synergy between the drugs, so they should be given together" not the answer?

The flattening reflects antagonism through induction and not synergy.

Why is "Beta-lactamase production" not the answer?

Beta-lactamase production is shown by penicillin zone edge appearance, and not by a D-test.

What concept does this question test?

It tests Clinical microbiology: D-test in Microbiology. The inducible phenotype is not seen with clindamycin alone, so the D-test unmasks it, and clindamycin may fail in vivo through selection of constitutive mutants.

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: D-test?

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.