For which condition is acyclovir most clearly indicated?

Acyclovir is an antiviral medication primarily used to treat infections caused by certain types of viruses, particularly the herpes simplex virus and varicella-zoster virus. Herpes zoster, commonly known as shingles, is caused by the reactivation of the varicella-zoster virus, which is the same virus that causes chickenpox.

In the case of the 77-year-old patient with herpes zoster infection, acyclovir is clearly indicated because it can reduce the duration and severity of the disease. Additionally, early treatment with acyclovir can help alleviate pain associated with shingles and may reduce the risk of complications such as postherpetic neuralgia.

The other conditions listed do not warrant acyclovir use. Hepatitis A and C are viral infections that are not treated with acyclovir, as they are unrelated to the herpes virus family. Respiratory syncytial virus (RSV) is a different viral infection that is typically managed with supportive care or specific antiviral treatments like ribavirin, rather than acyclovir, which targets herpes viruses.