Question

A young boy comes with a history of fever and cough since two weeks. He has reduced appetite and weight loss. He is also a known case of congenital heart disease, for which he has taken prophylaxis with penicillin. Sputum examination is normal; however, bronchoscopic lavage cultured on sheep blood agar showed evidence of gram-positive, aerobic, weakly acid-fast branching filaments. What is the likely diagnosis?

• A. Actinomycosis
• B. Nocardiosis
• C. Aspergillosis
• D. Mycobacterium tuberculosis

Hint:

Aerobic, branching, weakly acid-fast filaments equal Nocardia.

Answer 1

The Correct Option is B

This is a microbe-identification problem disguised as a clinical case, and the entire answer rests on three words from the culture report: aerobic, branching, and weakly acid-fast. The classic organism described by that exact triad is Nocardia, typically Nocardia asteroides, which produces a subacute pulmonary infection with fever, cough, and weight loss. Its partial acid-fastness is shown using a modified acid-fast stain because, unlike true mycobacteria, it decolourises with weaker acid. The closest mimic is Actinomyces, which is also a filamentous branching gram-positive organism, but Actinomyces is strictly anaerobic and is completely non-acid-fast, so the aerobic growth on blood agar and the weak acid-fastness rule it out. Mycobacterium tuberculosis is strongly acid-fast yet does not grow as branching filaments, eliminating that option, and Aspergillus is a mould with septate hyphae that would never stain gram-positive or acid-fast. The clinical backdrop adds support, since this child on penicillin prophylaxis would not be protected against Nocardia, an organism best treated with sulfonamides such as cotrimoxazole. Putting the staining characteristics together, the diagnosis is nocardiosis. \[\boxed{\text{Nocardiosis}}\]