Bacteriology: Mycobacterium marinum: 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 · Bacteriology: Mycobacterium marinum

Sample item

A man who cleans a home fish aquarium has a purple nodule on the hand that has spread as a line of nodules up the forearm over four weeks. Smears show acid-fast bacilli. Which laboratory feature identifies the organism?

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

The answer and the reasoning

C. Growth at 25 to 32 degrees C with yellow pigment after light exposure

Mycobacterium marinum causes swimming pool or fish tank granuloma, a sporotrichoid skin infection at cooler body sites. It is a photochromogen, producing yellow pigment only after exposure to light, and grows best at 28 to 32 degrees C, so culture is incubated at a lower temperature. Treatment is with combinations such as clarithromycin plus ethambutol.

Option by option

  • A. Failure to grow on any medium, even at low temperature

    Mycobacterium leprae is the mycobacterium that cannot be cultured, whereas M. marinum is culturable.

  • B. Obligate aerobic growth on blood agar in 24 hours

    Mycobacteria grow slowly and do not form colonies on blood agar within 24 hours.

  • C. Growth at 25 to 32 degrees C with yellow pigment after light exposure (correct)

    A cool-temperature, photochromogenic mycobacterium fits M. marinum.

  • D. Growth only at 45 degrees C with no pigment

    Growth at 45 degrees C describes thermophiles and does not fit this organism.

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, Growth at 25 to 32 degrees C with yellow pigment after light exposure. Mycobacterium marinum causes swimming pool or fish tank granuloma, a sporotrichoid skin infection at cooler body sites.

Why is "Failure to grow on any medium, even at low temperature" not the answer?

Mycobacterium leprae is the mycobacterium that cannot be cultured, whereas M. marinum is culturable.

Why is "Obligate aerobic growth on blood agar in 24 hours" not the answer?

Mycobacteria grow slowly and do not form colonies on blood agar within 24 hours.

Why is "Growth only at 45 degrees C with no pigment" not the answer?

Growth at 45 degrees C describes thermophiles and does not fit this organism.

What concept does this question test?

It tests Bacteriology: Mycobacterium marinum in Microbiology. It is a photochromogen, producing yellow pigment only after exposure to light, and grows best at 28 to 32 degrees C, so culture is incubated at a lower temperature. Treatment is with combinations such as clarithromycin plus ethambutol.

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 Bacteriology: Mycobacterium marinum?

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.