**Core Concept**
Hypopyon ulcer is a type of corneal ulcer that occurs in the presence of a hypopyon, which is a collection of pus or leukocytes in the anterior chamber of the eye. This condition is often associated with severe ocular infections.

**Why the Correct Answer is Right**
The formation of a hypopyon ulcer is thought to be due to the breakdown of the corneal epithelium and the subsequent infiltration of leukocytes into the cornea. This process is often facilitated by the presence of a severe ocular infection, such as keratitis or endophthalmitis, which can lead to the accumulation of pus in the anterior chamber. The hypopyon acts as a source of infection, causing further inflammation and tissue damage in the cornea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the formation of a hypopyon ulcer. While corneal ulcers can be caused by a variety of factors, the presence of a hypopyon is a specific characteristic of this condition.

**Option B:** This option is incorrect because it does not specify the presence of a hypopyon, which is a key factor in the formation of a hypopyon ulcer.

**Option C:** This option is incorrect because it is too broad and does not specifically address the formation of a hypopyon ulcer.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hypopyon ulcers are often associated with severe ocular infections, and prompt treatment with antibiotics and anti-inflammatory agents is crucial to prevent further damage to the cornea and vision.

**Correct Answer: B. Fungal keratitis.**