A fungal specimen from bone marrow of a patient with pulmonary infection shows tuberculate macroconidia at 30C and yeast-like cells at 36C. Identify the most likely dimorphic fungus.

A fungal specimen from bone marrow of a patient with pulmonary infection shows tuberculate macroconidia at 30C and yeast-like cells at 36C. Identify the most likely dimorphic fungus.

• A. Histoplasma capsulatum
• B. Blastomyces dermatitidis
• C. Coccidioides immitis
• D. Paracoccidioides brasiliensis

Answer: (A)

Dimorphism in Histoplasma capsulatum is demonstrated by a mold form that grows at environmental temperatures (about 25–30°C) producing tuberculate macroconidia, and a yeast form that appears at body temperature (around 37°C) as small intracellular yeast within macrophages. Seeing tuberculate macroconidia at 30°C and yeast-like cells at 36°C fits this organism’s two-stage life cycle. The bone marrow involvement with intracellular yeast strongly points to disseminated histoplasmosis in a pulmonary infection, which is a classic scenario for Histoplasma. The other fungi have distinct tissue morphologies—Blastomyces shows broad-based budding yeast in tissue, Coccidioides forms spherules with endospores, and Paracoccidioides often shows multiple budding yeast in a “pilot wheel” pattern—so those patterns don’t match the tuberculate macroconidia plus intracellular yeast seen here.

Dimorphism in Histoplasma capsulatum is demonstrated by a mold form that grows at environmental temperatures (about 25–30°C) producing tuberculate macroconidia, and a yeast form that appears at body temperature (around 37°C) as small intracellular yeast within macrophages. Seeing tuberculate macroconidia at 30°C and yeast-like cells at 36°C fits this organism’s two-stage life cycle. The bone marrow involvement with intracellular yeast strongly points to disseminated histoplasmosis in a pulmonary infection, which is a classic scenario for Histoplasma. The other fungi have distinct tissue morphologies—Blastomyces shows broad-based budding yeast in tissue, Coccidioides forms spherules with endospores, and Paracoccidioides often shows multiple budding yeast in a “pilot wheel” pattern—so those patterns don’t match the tuberculate macroconidia plus intracellular yeast seen here.