Which medication is ineffective against cytomegalovirus due to its lack of the necessary activating enzyme?

Cytomegalovirus (CMV) is a member of the herpesvirus family that requires specific antiviral medications for effective treatment. The medications used against CMV, such as ganciclovir, work by inhibiting viral DNA synthesis; however, this inhibition relies on the presence of specific activating enzymes produced by the virus.

Acyclovir is primarily effective against herpes simplex virus (HSV) and varicella-zoster virus (VZV) due to the presence of the viral thymidine kinase, which activates acyclovir into its active form. CMV lacks the necessary activating enzymes to convert acyclovir into its active form, rendering it ineffective against this virus.

The other medications listed, such as valacyclovir, ganciclovir, and famciclovir, target other herpesviruses or are specifically formulated to combat CMV (as with ganciclovir). Ganciclovir is effective against CMV due to its activation by viral enzymes specific to CMV. Valacyclovir, a prodrug of acyclovir, is similarly reliant on viral activation, while famciclovir also serves a similar role against another range of herpesviruses.

Thus, acyclovir's