Bacteriology: Mycobacterium kansasii: 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 kansasii

Sample item

A 48-year-old man with chronic obstructive lung disease has chronic cough and an upper-lobe cavity resembling tuberculosis. Sputum smear shows acid-fast bacilli. On Lowenstein-Jensen medium at 37 degrees C, slow-growing colonies are cream coloured in the dark and turn bright yellow after exposure to light. Which organism is this?

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

The answer and the reasoning

B. Mycobacterium kansasii

Mycobacterium kansasii is a photochromogen, producing pigment only after light exposure, and causes pulmonary disease that closely mimics tuberculosis. It is treated with rifampicin, isoniazid and ethambutol.

Option by option

  • A. Mycobacterium avium complex

    M. avium complex is non-chromogenic and typically produces non-pigmented colonies.

  • B. Mycobacterium kansasii (correct)

    Correct. Light-induced yellow pigment in a slow grower causing cavitary lung disease is M. kansasii.

  • C. Mycobacterium fortuitum

    M. fortuitum is a rapid grower that appears within about a week and does not match this slow growth.

  • D. Mycobacterium scrofulaceum

    M. scrofulaceum is a scotochromogen, pigmented even in the dark, and mainly causes cervical lymphadenitis in children.

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 B, Mycobacterium kansasii. Mycobacterium kansasii is a photochromogen, producing pigment only after light exposure, and causes pulmonary disease that closely mimics tuberculosis.

Why is "Mycobacterium avium complex" not the answer?

M. avium complex is non-chromogenic and typically produces non-pigmented colonies.

Why is "Mycobacterium fortuitum" not the answer?

M. fortuitum is a rapid grower that appears within about a week and does not match this slow growth.

Why is "Mycobacterium scrofulaceum" not the answer?

M. scrofulaceum is a scotochromogen, pigmented even in the dark, and mainly causes cervical lymphadenitis in children.

What concept does this question test?

It tests Bacteriology: Mycobacterium kansasii in Microbiology. It is treated with rifampicin, isoniazid and 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 kansasii?

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.