Histoplasmosis — DTM&H MCQ

Correct answer: C — Histoplasmosis

The correct answer is C, Histoplasmosis. The clinical picture (cave exposure in a bat-endemic area of Southeast Asia, subacute fever, dry cough, headache, and bilateral hilar lymphadenopathy on chest x-ray) is the classic presentation of acute pulmonary histoplasmosis following inhalation of Histoplasma capsulatum spores aerosolised from bat or bird guano in cave environments. The key laboratory clue, a dimorphic fungus that grows as a mould at ambient temperature but converts to a budding yeast form at 37C, is the defining mycological feature of Histoplasma capsulatum, which undergoes thermally regulated dimorphism controlled by temperature-dependent gene expression. This combination of cave exposure and hilar lymphadenopathy on imaging strongly favours histoplasmosis over the other endemic dimorphic mycoses, none of which share this specific epidemiological and radiological pairing. Why the other options are wrong: D. Blastomycosis: caused by Blastomyces dermatitidis, endemic to the Americas and parts of Africa (not Southeast Asia), typically linked to soil and decaying wood exposure rather than caves, with skin and bone involvement more prominent. A. Coccidioidomycosis: caused by Coccidioides species, restricted to arid soils of the southwestern United States and parts of Central and South America, acquired by inhaling arthroconidia from dust, not cave guano, and it is not a yeast-converting dimorphic organism in the same manner. B. Paracoccidioidomycosis: caused by Paracoccidioides brasiliensis, confined to rural Latin America, associated with agricultural soil exposure rather than caves, and often presents with mucocutaneous ulceration. E. Cryptococcosis: caused by Cryptococcus neoformans/gattii, a yeast at all temperatures (not dimorphic mould to yeast conversion), classically presents with meningitis in immunocompromised hosts and lacks the hilar lymphadenopathy pattern described. Key point: Cave exposure with bilateral hilar lymphadenopathy plus a mould that converts to yeast at 37C is pathognomonic for Histoplasma capsulatum infection.

Reference: Manson's Tropical Diseases, 24th edition, Chapter on Fungal Infections (Histoplasmosis): Histoplasma capsulatum is described as a thermally dimorphic fungus, mycelial at ambient temperature and yeast form at 37C, acquired by inhalation of spores from soil contaminated with bat or bird droppings, notably in caves.