**Core Concept**
Systemic acyclovir is an antiviral medication used to treat herpes zoster (shingles) by inhibiting viral DNA replication. This is achieved through its action on the viral enzyme thymidine kinase and DNA polymerase.

**Why the Correct Answer is Right**
Acyclovir works by being phosphorylated to its active form, acyclovir triphosphate, which then inhibits viral DNA polymerase, preventing the replication of herpes zoster virus. This results in a reduction in the severity and duration of the rash, pain, and other symptoms associated with herpes zoster. The use of systemic acyclovir in herpes zoster is particularly beneficial in immunocompromised patients or those with severe disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because acyclovir is not used to treat the underlying cause of herpes zoster, which is reactivation of latent varicella-zoster virus.
**Option B:** This option is incorrect because topical acyclovir is not effective in treating systemic herpes zoster, as it does not penetrate to the affected areas in sufficient concentrations.
**Option C:** This option is incorrect because valacyclovir is a prodrug of acyclovir, but systemic acyclovir itself is the correct answer, not its prodrug.

**Clinical Pearl / High-Yield Fact**
The use of systemic acyclovir in herpes zoster is most beneficial when started within 72 hours of the onset of symptoms, as this can significantly reduce the duration of pain and the severity of the rash.

**Correct Answer:** C. Valacyclovir is the prodrug of acyclovir and is used for the treatment of herpes zoster.