Which fungus listed is non-thermally dimorphic?

• A. H. capsulatum
• B. B. dermatidis
• C. P. marneffei
• D. C. immitis

Answer: (D)

Dimorphism in fungi refers to two forms that switch with temperature: the mold form in the environment (around 25C) and a yeast-like or tissue form at body temperature (37C). The classic thermally dimorphic pathogens switch to yeast in humans, which is how they cause disease.

Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides marneffei all exhibit the mold form in the environment and yield a true yeast form in human tissue at 37C, fitting the typical concept of thermally dimorphic fungi. Coccidioides immitis, however, does not produce a yeast form in tissue; it forms spherules containing endospores instead. Because its tissue stage is not a yeast, it is not considered to display the classic yeast-form thermally driven dimorphism, making it the non-thermally dimorphic option among the choices. In tissue you’d see large spherules rather than yeast cells.