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

Step 1: Extract the defining microbiological clues from the lavage: gram-positive, aerobic, weakly acid-fast branching filaments.
Step 2: A weakly (partially) acid-fast, AEROBIC, branching filamentous gram-positive organism is the textbook description of Nocardia. Modified acid-fast stains such as the modified Ziehl-Neelsen (Kinyoun) stain demonstrate this partial acid-fastness.
Step 3: Distinguish Nocardia from Actinomyces. Actinomyces is also a gram-positive branching filament but it is ANAEROBIC and NOT acid-fast, so the aerobic and acid-fast clues exclude Actinomyces (option A).
Step 4: Mycobacterium tuberculosis is strongly acid-fast but does not form branching filaments and is not described as a branching filament on blood agar, so option D is excluded. Aspergillus is a fungus with septate hyphae, not a gram-positive acid-fast bacterium, so option C is excluded.
Step 5: The penicillin prophylaxis (which does not cover Nocardia) and the subacute pulmonary picture fit. The diagnosis is Nocardiosis, option B.