Which culture medium is most appropriate for primary isolation of Histoplasma capsulatum from a sputum specimen of an immunocompromised patient?

• A Buffered charcoal yeast extract (BCYE) agar at 35°C
• B Middlebrook 7H10 agar at 37°C
• C Sabouraud dextrose agar with cycloheximide at 25°C ✓
• D Thayer-Martin agar at 37°C in 5% CO2

Explanation

Histoplasma capsulatum is a dimorphic fungus isolated in its mycelial (mould) form at room temperature (25–30°C). Sabouraud dextrose agar with cycloheximide (which inhibits most saprophytic fungi but not Histoplasma) is suitable for primary isolation from contaminated specimens. Growth is slow (2–6 weeks); colonies are white then brown with microconidia and characteristic tuberculate macroconidia. BCYE is used for Legionella; Middlebrook for mycobacteria; Thayer-Martin for Neisseria gonorrhoeae.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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