**Core Concept**
CMV (Cytomegalovirus) retinitis is a viral infection that occurs in immunocompromised individuals, particularly those with advanced HIV infection. The development of CMV retinitis is closely linked to the level of immunosuppression, which is reflected by the CD4 cell count.

**Why the Correct Answer is Right**
CMV retinitis typically occurs when the CD4 count falls below 100 cells/μL. This is because the CD4 cells, also known as T-helper cells, play a crucial role in coordinating the immune response against viral infections, including CMV. When the CD4 count is severely depleted, the individual is unable to mount an effective immune response against CMV, leading to the development of retinitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CMV retinitis can occur at CD4 counts above 100 cells/μL, although it is more common at lower CD4 counts.
**Option B:** This option is incorrect because it is not a commonly cited threshold for the development of CMV retinitis.
**Option C:** This option is incorrect because it is actually a higher threshold than the correct answer, and CMV retinitis can occur below this level.

**Clinical Pearl / High-Yield Fact**
In HIV-infected individuals, a CD4 count below 100 cells/μL is a significant risk factor for the development of opportunistic infections, including CMV retinitis. This highlights the importance of regular ophthalmological screening in HIV-infected patients with advanced immunosuppression.

**Correct Answer: A. 100 cells/μL.**