**Core Concept**
Fungal keratitis is a serious infection of the cornea that can lead to corneal ulceration and vision loss. **Fungal infections** of the cornea are often caused by **Aspergillus** and **Candida** species. The diagnosis and treatment of fungal keratitis require prompt attention to prevent complications.

**Why the Correct Answer is Right**
The correct answer is related to the most common cause of fungal corneal ulcers. **Aspergillus** species are a leading cause of fungal keratitis, particularly in tropical and subtropical regions. This is due to the high prevalence of **Aspergillus** in the environment and its ability to infect the cornea through trauma or other means.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although **Candida** can cause fungal keratitis, it is not the most common cause of fungal corneal ulcers.
**Option B:** This option is incorrect because it is not a common cause of fungal keratitis.
**Option C:** This option is incorrect because, although it can cause fungal infections, it is not typically associated with corneal ulcers.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the signs and symptoms of fungal keratitis, including a white or gray corneal infiltrate, to initiate prompt treatment and prevent vision loss.

**Correct Answer:** D. Aspergillus.