**Core Concept**
CMV retinitis is a sight-threatening opportunistic infection in HIV/AIDS patients, caused by the reactivation of cytomegalovirus (CMV) in the retina. This occurs when the host's cellular immunity, particularly the CD4+ T lymphocyte count, is severely compromised.

**Why the Correct Answer is Right**
CMV retinitis typically develops when the CD4 count falls below 100 cells/μL, marking a significant decline in the patient's immunocompetence. In such scenarios, the host's inability to mount an effective immune response against CMV allows the virus to reactivate and replicate within the retina, leading to characteristic retinal necrosis and vision loss. The pathogenesis of CMV retinitis involves the direct effect of CMV on retinal cells, as well as an indirect effect mediated by the host's inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CMV retinitis has been observed to occur at CD4 counts as low as 50 cells/μL, but not typically above 100 cells/μL. However, the exact threshold is not a fixed value and can vary among individuals.
**Option B:** This option is incorrect because CMV retinitis is not solely a consequence of the CD4 count; other factors such as the presence of antiretroviral therapy (ART) and the patient's overall clinical status also play a significant role.

**Clinical Pearl / High-Yield Fact**
A CD4 count below 100 cells/μL is a critical threshold for the development of opportunistic infections, including CMV retinitis, in HIV/AIDS patients. Early detection and treatment of CMV retinitis are essential to prevent vision loss and improve patient outcomes.

**Correct Answer: C. 100 cells/μL.**