**Core Concept:** Hypopyon is a yellowish-white, raised, and translucent accumulation of inflammatory cells, protein, and debris in the anterior chamber of the eye. It is typically seen in ocular infections, particularly fungal corneal ulcers. The presence of hypopyon signifies severe inflammation and indicates a poor prognosis if left untreated.

**Why the Correct Answer is Right:** Hypopyon is primarily composed of polymorphonuclear leukocytes (PMNs), also known as neutrophils, which are a type of white blood cell responsible for the immune response against bacterial infections. In fungal infections, the immune response involves eosinophils, which are also white blood cells. Since fungal corneal ulcers involve eosinophils, not PMNs, the correct answer is D: **Hypopyon in a fungal corneal ulcer contains eosinophils**.

**Why Each Wrong Option is Incorrect:**

A. **Neutrophils:** Neutrophils are involved in the immune response against bacterial infections, not fungal infections. They are not present in hypopyon in a fungal corneal ulcer.

B. **Mast cells:** Mast cells are involved in allergic reactions and anaphylaxis, not in the immune response against fungal infections. They are not present in hypopyon in a fungal corneal ulcer.

C. **Lymphocytes:** Lymphocytes are involved in cell-mediated immunity and adaptive immunity, not in the immediate immune response against fungal infections. They are not present in hypopyon in a fungal corneal ulcer.

**Clinical Pearl:** Hypopyon in an ocular infection signifies a severe inflammatory response, which might indicate a severe infection or an underlying systemic disorder affecting the eye. Prompt treatment is crucial to prevent potential complications and maintain eye health.