Bacteriology: Nocardia versus Actinomyces: 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: Nocardia versus Actinomyces

Sample item

A renal transplant recipient has a lung cavity and a brain abscess. Pus shows thin, beaded, branching Gram-positive filaments that stain with modified Ziehl-Neelsen (1% sulphuric acid) and grow aerobically. Which organism and drug of choice fit?

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

The answer and the reasoning

D. Nocardia asteroides complex, treated with trimethoprim-sulfamethoxazole

Nocardia is an aerobic, weakly (partially) acid-fast branching bacillus that causes pulmonary and brain abscesses in immunosuppressed hosts, and sulfonamides such as trimethoprim-sulfamethoxazole are the treatment of choice. Actinomyces is an anaerobic, non-acid-fast organism forming sulphur granules in cervicofacial, thoracic and pelvic disease, and responds to penicillin.

Option by option

  • A. Actinomyces israelii, treated with trimethoprim-sulfamethoxazole

    Actinomyces is anaerobic and not acid-fast, so the organism does not fit.

  • B. Nocardia asteroides complex, treated with high-dose penicillin G

    The organism is correct, but penicillin is the drug for Actinomyces and is unreliable for Nocardia.

  • C. Actinomyces israelii, treated with high-dose penicillin G

    Actinomyces is not acid-fast and is treated with penicillin, so both parts of this pairing are mismatched.

  • D. Nocardia asteroides complex, treated with trimethoprim-sulfamethoxazole (correct)

    Aerobic, partially acid-fast filaments with brain abscess indicate Nocardia, which is treated with a sulfonamide.

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 D, Nocardia asteroides complex, treated with trimethoprim-sulfamethoxazole. Nocardia is an aerobic, weakly (partially) acid-fast branching bacillus that causes pulmonary and brain abscesses in immunosuppressed hosts, and sulfonamides such as trimethoprim-sulfamethoxazole are the treatment of choice.

Why is "Actinomyces israelii, treated with trimethoprim-sulfamethoxazole" not the answer?

Actinomyces is anaerobic and not acid-fast, so the organism does not fit.

Why is "Nocardia asteroides complex, treated with high-dose penicillin G" not the answer?

The organism is correct, but penicillin is the drug for Actinomyces and is unreliable for Nocardia.

Why is "Actinomyces israelii, treated with high-dose penicillin G" not the answer?

Actinomyces is not acid-fast and is treated with penicillin, so both parts of this pairing are mismatched.

What concept does this question test?

It tests Bacteriology: Nocardia versus Actinomyces in Microbiology. Actinomyces is an anaerobic, non-acid-fast organism forming sulphur granules in cervicofacial, thoracic and pelvic disease, and responds to penicillin.

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: Nocardia versus Actinomyces?

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.