**Core Concept**
The question is testing the association between HIV and opportunistic retinal infections. **Cytomegalovirus (CMV) retinitis** is a common opportunistic infection in individuals with advanced HIV infection, leading to vision loss if left untreated.

**Why the Correct Answer is Right**
CMV retinitis occurs due to the reactivation of latent CMV infection in the retina. The virus infects retinal pigment epithelial cells and causes necrosis of the retina, leading to retinal hemorrhages, exudates, and vision loss. **CMV retinitis is the most common cause of blindness in individuals with AIDS**. The infection is often diagnosed in patients with a CD4 count below 100 cells/μL.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxoplasmosis is another opportunistic infection associated with HIV, but it primarily causes focal necrotizing retinitis, not the diffuse retinitis seen in CMV retinitis.

**Option B:** Herpes simplex virus (HSV) retinitis is a rare opportunistic infection in HIV-positive patients, typically presenting with a more localized retinal lesion.

**Option C:** Candida endophthalmitis is a fungal infection that can occur in immunocompromised patients, including those with HIV, but it is not the most common opportunistic retinal infection associated with HIV.

**Clinical Pearl / High-Yield Fact**
CMV retinitis is a medical emergency that requires prompt treatment with antiviral medications, such as ganciclovir or foscarnet, to prevent vision loss.

**Correct Answer:** C. CMV retinitis