**Core Concept**
Acyclovir is an antiviral medication that belongs to the class of nucleoside analogs. It works by inhibiting viral DNA replication, specifically targeting herpes simplex virus (HSV), varicella-zoster virus (VZV), and Epstein-Barr virus (EBV).

**Why the Correct Answer is Right**
Acyclovir is indicated in the treatment of herpes simplex encephalitis, herpes zoster (shingles), and herpes labialis (cold sores). It is also used to prevent cytomegalovirus (CMV) disease in immunocompromised patients, such as those with HIV/AIDS or undergoing organ transplantation. Acyclovir achieves this by incorporating into viral DNA and inhibiting viral DNA polymerase, which is necessary for viral replication.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because acyclovir is not indicated for the treatment of influenza virus infections, which are typically treated with oseltamivir or zanamivir.
**Option B:** This option is incorrect because acyclovir is not indicated for the treatment of respiratory syncytial virus (RSV) infections, which are typically treated with palivizumab in high-risk patients.
**Option C:** This option is incorrect because acyclovir is not indicated for the treatment of norovirus infections, which are typically treated with supportive care and hydration.

**Clinical Pearl / High-Yield Fact**
It is essential to note that acyclovir is not effective against all types of herpes viruses, and resistance can develop with prolonged use. Additionally, acyclovir is not a substitute for vaccination against herpes zoster (shingles).

**Correct Answer:** D.