**Core Concept**
Cytomegalovirus (CMV) retinitis is a common opportunistic infection in patients with acquired immunodeficiency syndrome (AIDS), particularly those with a CD4 count below 100 cells/μL. This condition is characterized by retinal necrosis and hemorrhage, often leading to vision loss.

**Why the Correct Answer is Right**
Cytomegalovirus is a herpesvirus that infects the retinal pigment epithelium and retinal capillaries, causing necrosis and inflammation. The primary mechanism of pathogenesis involves the replication of CMV in the retinal endothelial cells, leading to vascular occlusion and subsequent retinal damage. The immunocompromised state in AIDS patients makes them more susceptible to CMV retinitis.

**Why Each Wrong Option is Incorrect**
**Option B:** Toxoplasma gondii is a protozoan parasite that can cause ocular toxoplasmosis, typically characterized by focal necrotizing retinitis. However, it is not the primary cause of chorioretinitis in AIDS patients.

**Option C:** Cryptococcus neoformans is a fungus that can cause cryptococcal meningitis in immunocompromised individuals. While it can infect the eye, it is not typically associated with chorioretinitis in AIDS patients.

**Option D:** Histoplasma capsulatum is a fungus that can cause histoplasmosis, a pulmonary infection that can disseminate to other parts of the body, including the eye. However, it is not a common cause of chorioretinitis in AIDS patients.

**Clinical Pearl / High-Yield Fact**
In patients with AIDS, CMV retinitis is often associated with a characteristic "pizza pie" appearance on fundus examination, characterized by a combination of retinal hemorrhages, necrosis, and vascular occlusion.

**Correct Answer:**
✓ Correct Answer: A. Cytomegalovirus