**Core Concept**
CMV retinitis is a serious viral infection affecting individuals with compromised immune systems, such as those with HIV/AIDS or undergoing immunosuppressive therapy. It is caused by the cytomegalovirus (CMV) and can lead to vision loss if left untreated. Effective treatment involves antiviral medications that target the viral replication process.

**Why the Correct Answer is Right**
The correct answer is **Ganciclovir (Cytovene)**, which is a nucleoside analogue that inhibits viral DNA synthesis by acting as a chain terminator during viral replication. Ganciclovir is a first-line treatment for CMV retinitis, particularly in patients with HIV/AIDS or those undergoing immunosuppressive therapy. It is administered intravenously or orally and is effective in reducing viral replication and preventing further vision loss.

**Why Each Wrong Option is Incorrect**
**Option A:** **Foscarnet** is used in treating CMV retinitis, especially in cases resistant to ganciclovir or in patients with renal impairment. It works by directly inhibiting the viral DNA polymerase enzyme.
**Option B:** **Cidofovir** is also used in the treatment of CMV retinitis, particularly in patients with HIV/AIDS or those undergoing immunosuppressive therapy. It is a nucleotide analogue that inhibits viral DNA synthesis.
**Option D:** **Valganciclovir** is an oral prodrug of ganciclovir, used to treat CMV retinitis in patients with HIV/AIDS or those undergoing immunosuppressive therapy.

**Clinical Pearl / High-Yield Fact**
When treating CMV retinitis, it is essential to monitor for potential side effects, including neutropenia, thrombocytopenia, and nephrotoxicity. Regular ophthalmologic examinations are also crucial to assess the progression of the disease and adjust treatment as necessary.

**Correct Answer:** C. Foscarnet is not typically used as a first-line treatment for CMV retinitis.