## Core Concept
The question presents a case of an HIV-positive female with an indurated ulcer on the tongue, which suggests an opportunistic infection. The laboratory findings indicate a pathogen that can exhibit both hyphal and yeast forms under different conditions. This dimorphism is characteristic of certain fungi.

## Why the Correct Answer is Right
The description provided points towards an organism that can grow as hyphae at 20°C (or room temperature) on cornmeal agar and as budding yeasts at 37°C in human serum. This temperature-dependent dimorphism is a hallmark of *Candida albicans*, but the specific clue here—growth in cornmeal agar at 20°C showing hyphae and in human serum at 37°C showing budding yeasts—strongly suggests *Candida albicans*. *C. albicans* is an opportunistic pathogen that commonly causes infections in immunocompromised individuals, such as those with HIV/AIDS. It can cause oral candidiasis (thrush), which might present with ulcers.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific options provided, we can't directly address each, but we can infer based on common pathogens. For instance, if an option like *Aspergillus* were considered, it's incorrect because *Aspergillus* doesn't typically show budding yeasts at 37°C; it's more known for its hyphal growth.
- **Option B:** Similarly, if another option like *Histoplasma capsulatum* were considered, it's incorrect because it primarily exists as a yeast form at 37°C but doesn't typically show hyphal growth on cornmeal agar at 20°C in the context provided.
- **Option D:** If an option such as *Cryptococcus neoformans* were listed, it's incorrect because it doesn't exhibit hyphal growth; it's primarily known for its encapsulated yeast form.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Candida albicans* can exhibit a range of morphologies, including yeast, hyphae, and pseudohyphae, which is crucial for its identification and pathogenicity. In immunocompromised patients, *C. albicans* can cause a variety of infections, ranging from mucocutaneous to systemic infections.

## Correct Answer Line
**Correct Answer: C.**