A patient presents with a painful vesicular eruption of the hard palate consistent with herpes zoster. Which of the following antiviral agents is the appropriate choice?

Correct answer: C. Famciclovir

• A. GANCICLOVIR is reserved for CYTOMEGALOVIRUS — retinitis, colitis and pneumonitis in transplant recipients and patients with advanced HIV. It is far more myelosuppressive than the HSV/VZV agents (neutropenia, thrombocytopenia) and is not used for zoster.
• B. OSELTAMIVIR is a neuraminidase inhibitor for influenza A and B. It has no herpesvirus activity.
• C. CORRECT. FAMCICLOVIR (the oral prodrug of penciclovir) is licensed for herpes simplex and varicella-zoster infections, alongside acyclovir and valacyclovir. All three are guanosine analogues that are selectively phosphorylated by the VIRAL thymidine kinase, which is why they concentrate in infected cells and spare uninfected ones. For zoster, therapy must begin within 72 hours of rash onset to shorten the course and reduce the risk of post-herpetic neuralgia.
• D. LAMIVUDINE (3TC) is a nucleoside reverse transcriptase inhibitor used for HIV and chronic hepatitis B. It has no activity against the herpesviruses.

Note: Sort the '-vir' drugs by target: HSV/VZV → acyclovir, valacyclovir, famciclovir, penciclovir. CMV → ganciclovir, valganciclovir, foscarnet, cidofovir. HIV → lamivudine, zidovudine, abacavir, tenofovir. Influenza → oseltamivir, zanamivir. HCV → sofosbuvir. The shared '-vir' stem tells you it is an antiviral; it tells you nothing about which virus.