What is the alternative antiviral used if Ganciclovir fails against CMV retinitis?

The correct answer is Famciclovir, which is an antiviral medication used to treat infections caused by certain types of viruses, including cytomegalovirus (CMV) retinitis. When Ganciclovir, the first-line treatment for CMV retinitis, fails or is not tolerated by the patient, Famciclovir can serve as an effective alternative. It is primarily used for managing infections of the herpes virus family and has demonstrated some efficacy against CMV in specific cases.

Understanding why Famciclovir is the appropriate choice requires recognizing its mechanism of action. Famciclovir is a prodrug that gets metabolized into its active form, penciclovir, which inhibits viral DNA synthesis by interfering with the viral polymerase enzyme. This makes it potentially effective in managing cases where Ganciclovir is ineffective.

The other options do not serve as suitable alternatives for CMV retinitis. Acyclovir, while effective against other herpes viruses such as HSV and VZV, has limited efficacy against CMV. Amphotericin B and Nystatin are antifungal agents; thus, they target fungal infections rather than viral infections, making them incorrect choices for treating CMV retinitis.