## Core Concept
The question presents a case of a HIV-positive female with an indurated ulcer on the tongue, and laboratory findings that indicate a specific type of fungal infection. The key details are the growth characteristics of the organism at different temperatures and its microscopic morphology. This points towards a dimorphic fungus, which can exist as both hyphae and yeast forms under different conditions.

## Why the Correct Answer is Right
The organism in question grows as **hyphae** on cornmeal agar at 20°C (or room temperature), and as **budding yeasts** in human serum at 37°C (body temperature). This dimorphic characteristic is typical of certain pathogenic fungi. The description fits *Histoplasma capsulatum*, *Blastomyces dermatitidis*, *Coccidioides immitis*, and *Sporothrix schenckii*. However, *Sporothrix schenckii* is particularly known for causing skin ulcers (like the one described) through inoculation, and it grows as a mold at room temperature and as yeast at body temperature.

## 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 example, if *Candida* species were considered, they primarily exist as yeasts and don't typically show a dimorphic growth pattern as described.
- **Option B:** Similarly, without specifics, if another fungus doesn't exhibit the described temperature-dependent dimorphism, it would be incorrect.
- **Option D:** This would be incorrect for similar reasons as Option A and B, primarily if the organism doesn't match the described growth characteristics.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Sporothrix schenckii* infections are often associated with gardeners or individuals who work with soil or plants, as the fungus can enter the body through skin breaks. The classic presentation is a painless, indurated ulcer that may develop along lymphatic channels. In immunocompromised patients, like those with HIV/AIDS, the infection can disseminate.

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