**Core Concept**
Acyclovir is an antiviral medication used to treat herpes simplex virus (HSV) infections, including herpes zoster (shingles), by inhibiting viral DNA replication. Its mechanism of action involves the selective inhibition of viral DNA polymerase, an enzyme essential for viral replication.

**Why the Correct Answer is Right**
Acyclovir is the drug of choice for herpes zoster because it effectively reduces the duration and severity of pain associated with the condition. It works by selectively targeting the viral DNA polymerase enzyme, which is essential for the replication of the varicella-zoster virus (VZV). Acyclovir is most effective when initiated within 72 hours of the onset of rash, as it can significantly reduce the risk of postherpetic neuralgia (PHN).

**Why Each Wrong Option is Incorrect**
**Option A:** Valacyclovir is an antiviral medication that is a prodrug of acyclovir, meaning it is converted to acyclovir in the body. While it can be used to treat herpes zoster, it is not the primary drug of choice due to its higher cost and potential side effects.
**Option B:** Famciclovir is another antiviral medication that is used to treat herpes zoster, but it is not as effective as acyclovir in reducing the risk of PHN.
**Option C:** Penciclovir is an antiviral medication that is topical and not systemic, making it less effective for treating herpes zoster.

**Clinical Pearl / High-Yield Fact**
It's essential to initiate antiviral therapy within 72 hours of the onset of rash to maximize the effectiveness of treatment and reduce the risk of postherpetic neuralgia.

**Correct Answer: A. Acyclovir**