**Core Concept**
The underlying principle being tested is the identification of opportunistic infections in immunocompromised patients, specifically those with AIDS. **Cytomegalovirus (CMV)** is a common cause of infections in AIDS patients due to their compromised immune system.

**Why the Correct Answer is Right**
CMV is the most common cause of chorioretinitis in AIDS patients. The virus can reactivate in immunocompromised individuals, leading to **CMV retinitis**, which can cause blindness if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect as it is not the most common cause.
**Option B:** Incorrect, not typically associated with chorioretinitis in AIDS.
**Option C:** Incorrect, though a possibility, not the most common cause.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **CMV retinitis** is a medical emergency requiring prompt treatment to prevent vision loss.

**Correct Answer:** D. Cytomegalovirus.