Herpes resistant to acyclovir is treated by –
**Core Concept**
Herpes simplex virus (HSV) resistance to acyclovir is a clinical concern, particularly in immunocompromised patients. Acyclovir is a nucleoside analog that inhibits viral DNA synthesis by competing with guanine for viral DNA polymerase.
**Why the Correct Answer is Right**
HSV resistance to acyclovir often arises due to mutations in the viral thymidine kinase (TK) gene or the viral DNA polymerase gene. These mutations reduce the efficacy of acyclovir activation and incorporation into viral DNA, respectively. **Foscarnet**, a pyrophosphate analog, is a suitable alternative for treating acyclovir-resistant HSV infections. Foscarnet acts by directly inhibiting viral DNA polymerase, bypassing the need for TK activation, and is effective against acyclovir-resistant strains.
**Why Each Wrong Option is Incorrect**
**Option A:** **Valacyclovir** is a prodrug of acyclovir that is converted to acyclovir in the body, making it ineffective against acyclovir-resistant strains.
**Option B:** **Ganciclovir** is another antiviral drug that targets herpesviruses, but it is not the preferred treatment for acyclovir-resistant HSV.
**Option C:** **Ribavirin** is an antiviral drug that targets various viruses, including respiratory syncytial virus (RSV), but it is not effective against herpesviruses.
**Clinical Pearl / High-Yield Fact**
When treating herpes simplex virus infections, it is essential to consider the potential for resistance, particularly in immunocompromised patients. Foscarnet should be used as a second-line treatment for acyclovir-resistant HSV infections.
**Correct Answer:** C. Foscarnet