**Core Concept**
Endothrix is a type of fungal keratitis characterized by the invasion of fungal hyphae within the endothelial cells of the cornea. This condition is caused by a specific type of fungal infection that affects the corneal endothelium.

**Why the Correct Answer is Right**
Endothrix is typically caused by the fungus *Fusarium*, which enters the cornea through a break in the epithelium and invades the endothelial cells. This infection can lead to corneal edema, vision loss, and potentially severe complications if left untreated. The fungal hyphae within the endothelial cells can cause significant damage to the cornea, leading to endothelial dysfunction and vision impairment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because endothrix is not caused by bacteria. Bacterial keratitis is a different type of infection that affects the cornea.
**Option B:** This option is incorrect because endothrix is not caused by viruses. Viral keratitis is another type of infection that affects the cornea, but it is not associated with endothrix.
**Option C:** This option is incorrect because endothrix is not caused by protozoa. Protozoal keratitis is a rare type of infection that affects the cornea, but it is not associated with endothrix.

**Clinical Pearl / High-Yield Fact**
Endothrix is a type of fungal keratitis that requires prompt treatment to prevent vision loss. Fungal infections of the cornea can be notoriously difficult to treat, and early diagnosis is crucial to prevent complications.

**Correct Answer: A. Fusarium**