For which of the following conditions would acyclovir be least effective?

Acyclovir is an antiviral medication that is specifically effective against herpes virus infections, including herpes simplex and herpes zoster. The drug works by interfering with the replication of viral DNA, which is a key process in the life cycle of these herpes viruses.

When considering respiratory syncytial virus (RSV), it is important to note that acyclovir is not effective against this virus. RSV is a paramyxovirus, and its mechanism of infection and replication is different from that of herpes viruses. Consequently, acyclovir does not have a therapeutic effect on RSV, making it the least effective option among the listed conditions.

In contrast, acyclovir is successfully used to treat herpes simplex virus infections, such as genital herpes or cold sores, and also for herpes zoster, which is responsible for shingles. Additionally, while acyclovir can be used to some extent for cytomegalovirus infections, it is not the primary treatment since ganciclovir is preferred for that specific condition. Thus, while acyclovir has its roles, RSV is the condition for which it would be least effective.