Clinical microbiology and hospital infection control: colistin susceptibility testing: 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: colistin susceptibility testing

Sample item

A laboratory reports colistin susceptibility of a Klebsiella isolate using disc diffusion, and a reviewer objects. What is the correct reason?

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

The answer and the reasoning

C. Colistin diffuses poorly in agar, so broth microdilution is needed

Colistin is a large polycationic molecule that diffuses poorly in agar, so disc diffusion gives unreliable zones. Broth microdilution is the recommended reference method for MIC determination.

Option by option

  • A. Disc diffusion measures only bactericidal activity

    Disc diffusion does not measure bactericidal activity.

  • B. Colistin is inactivated by Mueller-Hinton agar

    Mueller-Hinton agar does not inactivate colistin.

  • C. Colistin diffuses poorly in agar, so broth microdilution is needed (correct)

    Correct. Poor diffusion makes disc results unreliable, so broth microdilution is used.

  • D. Klebsiella cannot be tested by any susceptibility method

    Klebsiella can be tested by standard methods for most drugs.

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 C, Colistin diffuses poorly in agar, so broth microdilution is needed. Colistin is a large polycationic molecule that diffuses poorly in agar, so disc diffusion gives unreliable zones.

Why is "Disc diffusion measures only bactericidal activity" not the answer?

Disc diffusion does not measure bactericidal activity.

Why is "Colistin is inactivated by Mueller-Hinton agar" not the answer?

Mueller-Hinton agar does not inactivate colistin.

Why is "Klebsiella cannot be tested by any susceptibility method" not the answer?

Klebsiella can be tested by standard methods for most drugs.

What concept does this question test?

It tests Clinical microbiology and hospital infection control: colistin susceptibility testing in Microbiology. Broth microdilution is the recommended reference method for MIC determination.

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: colistin susceptibility testing?

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.