An HIV positive female presents with an indurated ulcer over the tongue. Laboratory findings show growth in cornmeal agar at 20-degree Celsius, microscopy showing hyphae and growth in human serum at 37-degree Celsius show budding yeasts. The probable cause is –
**Core Concept**
The patient's presentation of an indurated ulcer over the tongue, along with laboratory findings of growth in cornmeal agar at 20°C showing hyphae and growth in human serum at 37°C showing budding yeasts, suggests a fungal infection that exhibits dimorphism. This is a characteristic feature of certain pathogenic fungi that can exist in both yeast and hyphal forms, depending on the environmental conditions.
**Why the Correct Answer is Right**
The growth in cornmeal agar at 20°C showing hyphae is typical of the mold form of the fungus, while the growth in human serum at 37°C showing budding yeasts indicates the yeast form. This dimorphic transformation is a key feature of *Histoplasma capsulatum*, *Blastomyces dermatitidis*, and *Coccidioides immitis*. However, in this scenario, the presence of an indurated ulcer over the tongue and the laboratory findings point towards *Histoplasma capsulatum*. The yeast form of *Histoplasma capsulatum* resembles *Cryptococcus neoformans* but can be differentiated by its ability to form a capsule. The patient's HIV-positive status makes her more susceptible to opportunistic infections, including fungal ones.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because *Aspergillus fumigatus* does not exhibit dimorphism and typically grows as a mold, not a yeast.
* **Option B:** This option is incorrect because *Candida albicans* can form pseudohyphae but does not show the characteristic dimorphism seen in *Histoplasma capsulatum*.
* **Option C:** This option is incorrect because *Coccidioides immitis* typically causes pulmonary infections, and the presentation of an indurated ulcer over the tongue is not typical of this fungus.
**Clinical Pearl / High-Yield Fact**
When encountering a patient with a fungal infection that exhibits dimorphism, consider *Histoplasma capsulatum* as a possible cause, especially if the patient has an underlying immunocompromised condition. The presence of an indurated ulcer over the tongue, along with laboratory findings, can aid in the diagnosis.
**Correct Answer:** D. *Histoplasma capsulatum*.