Which antiviral drug is used to prevent cytomegalovirus (CMV) in organ transplant patients?

Ganciclovir is specifically indicated for the prevention and treatment of cytomegalovirus (CMV) infections, particularly in immunocompromised patients such as those undergoing organ transplantation. After a transplant, patients are at a significantly increased risk for CMV infection due to their suppressed immune systems; therefore, antiviral prophylaxis is essential.

Ganciclovir works by inhibiting viral DNA synthesis, thus preventing the replication of the virus. Its efficacy in preventing CMV reactivation makes it the drug of choice in this scenario. While Valganciclovir, which is an oral prodrug of ganciclovir, is also used for the prophylaxis and treatment of CMV, ganciclovir itself is the primary drug referenced for this specific context of prevention in organ transplant patients.

The other antiviral agents listed do not target CMV. Acyclovir is primarily used for herpes simplex and varicella-zoster viruses. Oseltamivir is used for the treatment and prevention of influenza. Thus, the reasoning for selecting ganciclovir as the correct answer aligns with its established role in CMV management in high-risk populations.