A patient with AIDS develops pulmonary infiltrates and BAL shows broad-based budding yeast with double-contoured cell wall on KOH preparation. The organism grows as white to tan colonies on SDA at 25°C (mold phase) and converts to yeast at 37°C. Serology shows complement-fixing antibodies at 1:32. The diagnosis is:
- A Coccidioidomycosis (Coccidioides immitis) — has spherules with endospores, not yeast
- B Paracoccidioidomycosis (Paracoccidioides brasiliensis) — mariners wheel/pilot wheel appearance with multiple buds
- C Blastomycosis (Blastomyces dermatitidis) — broad-based budding yeast, thermally dimorphic ✓
- D Histoplasmosis (Histoplasma capsulatum) — small intracellular yeast within macrophages
Explanation
Blastomyces dermatitidis characteristically produces broad-based budding yeast (mother and daughter cell connected by a wide base, typically 8–15 μm) in tissue and direct microscopy, with a thick double-contoured cell wall. It is thermally dimorphic — mold at 25°C (delicate hyphae with conidia) and yeast at 37°C. In BAL, the broad-based bud is the diagnostic clue. Complement fixation serology supports diagnosis. Coccidioides produces spherules with endospores, not broad-based buds. Paracoccidioides produces multiple peripheral buds on a large mother cell (mariners wheel). Histoplasma produces small (2–4 μm) yeast cells found intracellularly within macrophages.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.