Which fungus is a classic example of a dimorphic pathogen that shifts between mold and yeast forms?

• A. Candida albicans
• B. Histoplasma capsulatum
• C. Aspergillus niger
• D. Coccidioides immitis

Answer: (D)

Dimorphic fungi show two forms depending on their environment: a mold form in the environment and a distinct tissue form inside a host. This switch is central to how they cause disease. Coccidioides immitis is a quintessential example: in soil it grows as mold with hyphae that produce arthroconidia; when inhaled and inside the body, it transforms into a tissue form that forms large spherules filled with endospores. This mold-to-spherule transition is the hallmark of a classic dimorphic pathogen and underlies the disease coccidioidomycosis (Valley fever). The other organisms listed don’t illustrate this classic environmental mold to tissue form switch in the same way.