General microbiology and sterilisation: cetrimide agar: 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 · General microbiology and sterilisation: cetrimide agar

Sample item

A swab from an infected burn wound grows a blue-green pigmented, oxidase-positive, non-lactose-fermenting bacillus. Which selective medium containing a quaternary ammonium compound is used to isolate this organism?

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

The answer and the reasoning

C. Cetrimide agar

Cetrimide inhibits most other bacteria and favours Pseudomonas aeruginosa, which also enhances pigment production. Bile aesculin is for enterococci and group D streptococci, Thayer-Martin for pathogenic Neisseria, and tellurite medium for Corynebacterium diphtheriae.

Option by option

  • A. Tellurite blood agar

    Tellurite medium is used for Corynebacterium diphtheriae, giving grey-black colonies.

  • B. Thayer-Martin medium

    Thayer-Martin is a selective chocolate agar for gonococci and meningococci.

  • C. Cetrimide agar (correct)

    Correct. Cetrimide agar is selective for Pseudomonas aeruginosa.

  • D. Bile aesculin agar

    Bile aesculin agar identifies enterococci by aesculin hydrolysis in the presence of bile.

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, Cetrimide agar. Cetrimide inhibits most other bacteria and favours Pseudomonas aeruginosa, which also enhances pigment production.

Why is "Tellurite blood agar" not the answer?

Tellurite medium is used for Corynebacterium diphtheriae, giving grey-black colonies.

Why is "Thayer-Martin medium" not the answer?

Thayer-Martin is a selective chocolate agar for gonococci and meningococci.

Why is "Bile aesculin agar" not the answer?

Bile aesculin agar identifies enterococci by aesculin hydrolysis in the presence of bile.

What concept does this question test?

It tests General microbiology and sterilisation: cetrimide agar in Microbiology. Bile aesculin is for enterococci and group D streptococci, Thayer-Martin for pathogenic Neisseria, and tellurite medium for Corynebacterium diphtheriae.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 General microbiology and sterilisation: cetrimide agar?

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.