**Core Concept**
Gancyclovir is an antiviral medication used to treat cytomegalovirus (CMV) infections. In the context of intravitreal administration, it is used to treat CMV retinitis, a serious viral infection that can cause vision loss in individuals with compromised immune systems, such as those with AIDS or undergoing immunosuppressive therapy.

**Why the Correct Answer is Right**
Intravitreal gancyclovir is administered directly into the vitreous humor, the gel-like substance that fills the center of the eyeball, to achieve high concentrations of the medication in the retina. This localized treatment allows for effective suppression of CMV replication, thereby preventing further retinal damage. The mechanism of action of gancyclovir involves the inhibition of viral DNA synthesis, which is essential for the replication of CMV.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Intravitreal gancyclovir is typically used in conjunction with systemic antiviral therapy, such as valgancyclovir, to treat CMV retinitis. It is essential to monitor patients for signs of toxicity, such as retinal detachment or vitreous hemorrhage, which can occur with intravitreal injections.

**Correct Answer:** C. CMV retinitis