**Core Concept**
The patient's symptoms and funduscopic examination findings are suggestive of cytomegalovirus (CMV) retinitis, a common opportunistic infection in individuals with acquired immunodeficiency syndrome (AIDS). CMV retinitis is characterized by progressive vision loss due to retinal necrosis caused by CMV.

**Why the Correct Answer is Right**
Ganciclovir is an antiviral medication that is effective against CMV. It works by inhibiting viral DNA synthesis, thereby preventing the replication of CMV. Ganciclovir is specifically taken up by CMV-infected cells, where it is phosphorylated to its active form, ganciclovir triphosphate. This active form then inhibits viral DNA polymerase, preventing the virus from replicating. The use of ganciclovir has been shown to improve vision and reduce the risk of blindness in patients with CMV retinitis.

**Why Each Wrong Option is Incorrect**

**Option A:** Acyclovir is an antiviral medication that is primarily effective against herpes simplex virus (HSV) and varicella-zoster virus (VZV). It is not effective against CMV and would not be the appropriate choice for treating CMV retinitis.

**Option B:** Amantadine is an antiviral medication that is primarily used to treat influenza A virus infections. It has no activity against CMV and would not be effective in treating CMV retinitis.

**Option C:** Flucytosine is an antifungal medication that is used to treat cryptococcal meningitis and other fungal infections. It has no activity against CMV and would not be effective in treating CMV retinitis.

**Clinical Pearl / High-Yield Fact**
CMV retinitis is a common opportunistic infection in individuals with AIDS, and prompt treatment with ganciclovir is essential to prevent vision loss. It is essential to note that CMV retinitis can occur in individuals with CD4 counts as high as 100 cells/μL, emphasizing the need for regular ophthalmologic examination in patients with AIDS.

**✓ Correct Answer: D. Ganciclovir. Ganciclovir.**