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

**Why the Correct Answer is Right**
Acyclovir is selectively phosphorylated by viral thymidine kinase (TK) to form acyclovir monophosphate, which is then further phosphorylated by cellular enzymes to form acyclovir triphosphate. Acyclovir triphosphate acts as a competitive inhibitor of viral DNA polymerase, preventing the elongation of viral DNA strands. This mechanism of action is specific to herpes viruses, making acyclovir an effective treatment for HSV and VZV infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because acyclovir is not primarily used for the treatment of influenza.
**Option B:** This option is incorrect because acyclovir is not a suitable treatment for bacterial infections such as methicillin-resistant Staphylococcus aureus (MRSA).
**Option C:** This option is incorrect because although acyclovir may have some immunomodulatory effects, it is not primarily used as an immunosuppressive agent.

**Clinical Pearl / High-Yield Fact**
Acyclovir is most effective when started within 72 hours of the onset of symptoms in patients with herpes simplex encephalitis.

**Correct Answer: C. Herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections.**