Question

A case of brain abscess reveals Gram-positive, branching, filamentous organisms on microscopy. The organism is weakly acid-fast and grows on paraffin bait media. Which is the most likely etiologic agent?

• A. Actinomyces israelii
• B. Mycobacterium tuberculosis
• C. Nocardia asteroides
• D. Cryptococcus neoformans

Hint:

Gram-positive branching filamentous organism with weak acid-fastness = Nocardia. Gram-positive branching filamentous organism without acid-fastness = Actinomyces.

Answer 1

The Correct Option is C

Concept: Nocardia is an aerobic, Gram-positive, branching filamentous bacterium. It is characteristically weakly acid-fast because of the presence of mycolic acids in its cell wall. It commonly causes pulmonary infection and can spread to the brain, producing brain abscesses.

Step 1: Identify the key microscopy clue.
The question mentions: \[ \text{Gram-positive + branching + filamentous organisms} \] This points toward either Actinomyces or Nocardia.

Step 2: Use acid-fastness to differentiate.
Actinomyces is not acid-fast, whereas Nocardia is weakly acid-fast. Since the organism is weakly acid-fast, the diagnosis strongly favors: \[ \textit{Nocardia asteroides} \]

Step 3: Connect with brain abscess.
Nocardia asteroides can cause lung infection and may disseminate hematogenously to the brain, causing brain abscess, especially in immunocompromised patients. Therefore, the most likely etiologic agent is {Nocardia asteroides.