An HIV positive female has an indurated ulcer over the tongue. Laboratory findings show growth in cornmeal agar at 20 degrees, microscopy showing hyphae and growth inhuman serum at 37 degrees show budding yeasts. The probable cause is:
**Core Concept**
The patient's symptoms and laboratory findings suggest an opportunistic infection caused by a microorganism that can exist in both yeast and hyphal forms. This phenomenon is characteristic of a type of fungi known as dimorphic fungi, which can switch between these two forms depending on the environment.
**Why the Correct Answer is Right**
The patient's symptoms of an indurated ulcer on the tongue, combined with the laboratory findings of growth in cornmeal agar at 20 degrees (which favors the growth of hyphae) and growth in human serum at 37 degrees (which favors the growth of budding yeasts), are consistent with an infection caused by *Histoplasma capsulatum*. This dimorphic fungus typically grows as a mold at room temperature, but converts to a yeast form at body temperature.
**Why Each Wrong Option is Incorrect**
***Option A:** This option is incorrect because *Candida albicans*, a common fungal pathogen, typically grows as a yeast at room temperature and does not exhibit the same kind of dimorphism as *Histoplasma capsulatum*.
***Option B:** This option is incorrect because *Aspergillus* species are typically molds that grow at room temperature and do not exhibit the same kind of dimorphism as *Histoplasma capsulatum*.
***Option C:** This option is incorrect because *Mucor* species are typically molds that grow at room temperature and do not exhibit the same kind of dimorphism as *Histoplasma capsulatum*.
**Clinical Pearl / High-Yield Fact**
Dimorphic fungi like *Histoplasma capsulatum* can cause a range of systemic infections, including histoplasmosis, which can be particularly severe in immunocompromised patients. It is essential to consider these pathogens in the differential diagnosis of opportunistic infections in HIV-positive patients.
**Correct Answer:** C. *Histoplasma capsulatum*.