**Core Concept**
Varicella-Zoster Virus (VZV) reactivation is a common complication in immunocompromised individuals, leading to herpes zoster or shingles. The primary goal of antiviral therapy is to reduce the severity and duration of symptoms, prevent postherpetic neuralgia, and decrease the risk of complications.

**Why the Correct Answer is Right**
Acyclovir is a nucleoside analog that selectively inhibits viral DNA synthesis by competing with deoxyguanosine triphosphate (dGTP) for incorporation into viral DNA. This results in viral DNA chain termination and ultimately, inhibition of viral replication. Acyclovir is effective in treating herpes zoster due to its ability to penetrate the central nervous system and reach therapeutic concentrations in the cerebrospinal fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** Foscarnet is a pyrophosphate analog that inhibits viral replication by binding to the viral DNA polymerase, but it is not the first-line treatment for herpes zoster and has a higher toxicity profile compared to acyclovir.
**Option B:** Ribavirin is a nucleoside analog that has broad-spectrum antiviral activity, but it is not specifically indicated for the treatment of herpes zoster and may have adverse effects on the bone marrow.
**Option C:** Oseltamivir is an adamantane derivative that selectively inhibits the influenza virus neuraminidase, and it is not effective against VZV.

**Clinical Pearl / High-Yield Fact**
Acyclovir is most effective when started within 72 hours of the onset of rash, and it can reduce the risk of postherpetic neuralgia by 50% when administered within this time frame.

**Correct Answer:** C. Acyclovir