Bacteriology: Rhodococcus equi: 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: Rhodococcus equi

Sample item

A man with AIDS (CD4 count 25 per microlitre) who works on a horse farm has fever and a cavitating upper lobe pneumonia. Sputum shows pleomorphic Gram-positive coccobacilli that are weakly acid-fast, and culture produces salmon-pink mucoid colonies. Which organism is this?

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

The answer and the reasoning

C. Rhodococcus equi

Rhodococcus equi is a soil and herbivore-associated actinomycete that causes cavitary pneumonia in severely immunocompromised patients. It is weakly acid-fast and forms pink mucoid colonies, whereas Nocardia is a branching filamentous organism.

Option by option

  • A. Nocardia asteroides

    Nocardia forms branching beaded filaments with chalky colonies, not coccobacilli.

  • B. Mycobacterium tuberculosis

    M. tuberculosis is strongly acid-fast and does not grow as mucoid pink colonies.

  • C. Rhodococcus equi (correct)

    Correct. Horse exposure, a weakly acid-fast coccobacillus and salmon-pink colonies indicate R. equi.

  • D. Actinomyces israelii

    A. israelii is a non-acid-fast anaerobe that forms sulphur granules.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Rhodococcus equi. Rhodococcus equi is a soil and herbivore-associated actinomycete that causes cavitary pneumonia in severely immunocompromised patients.

Why is "Nocardia asteroides" not the answer?

Nocardia forms branching beaded filaments with chalky colonies, not coccobacilli.

Why is "Mycobacterium tuberculosis" not the answer?

M. tuberculosis is strongly acid-fast and does not grow as mucoid pink colonies.

Why is "Actinomyces israelii" not the answer?

A. israelii is a non-acid-fast anaerobe that forms sulphur granules.

What concept does this question test?

It tests Bacteriology: Rhodococcus equi in Microbiology. It is weakly acid-fast and forms pink mucoid colonies, whereas Nocardia is a branching filamentous organism.

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 Bacteriology: Rhodococcus equi?

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.