**Core Concept**
The underlying principle being tested is the association between AIDS and opportunistic infections, particularly those affecting the eye. **Cytomegalovirus (CMV)** is a common cause of opportunistic infections in immunocompromised patients.

**Why the Correct Answer is Right**
CMV retinitis is the most common ocular infection in patients with AIDS, caused by the reactivation of latent CMV due to the compromised immune system. This leads to inflammation of the retina, which can cause blindness if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxoplasmosis is indeed an opportunistic infection, but it is not the most common ocular infection in AIDS patients.
**Option B:** Herpes simplex is a common cause of ocular infections, but not as prevalent in AIDS patients as CMV.
**Option D:** Fungal infections can occur in immunocompromised patients, but they are not the most common cause of ocular infections in AIDS.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that CMV retinitis is a significant cause of blindness in AIDS patients, and its diagnosis requires a high index of suspicion.

**Correct Answer:** C. CMV retinitis