Drug of choice for acyclovir resistant herpes is :
**Core Concept**
Acyclovir resistance in herpes simplex virus (HSV) occurs due to mutations in the viral thymidine kinase (TK) or DNA polymerase genes, leading to impaired activation or incorporation of the drug. The primary mechanism of action of acyclovir involves its conversion to acyclovir monophosphate by viral TK, which is then further phosphorylated to the active triphosphate form by host cell enzymes.
**Why the Correct Answer is Right**
Foscarnet is a potent antiviral agent that does not require activation by viral TK, thus bypassing the resistance mechanism. It acts by inhibiting viral DNA polymerase, preventing viral replication. Foscarnet is particularly effective against acyclovir-resistant strains of HSV and varicella-zoster virus (VZV). Its use is generally reserved for severe or complicated cases of herpes infections.
**Why Each Wrong Option is Incorrect**
**Option A:** Valacyclovir is an oral prodrug of acyclovir, which would be ineffective against acyclovir-resistant strains.
**Option B:** Cidofovir is another antiviral drug that targets viral DNA polymerase, but it is not the first-line choice for acyclovir-resistant herpes infections.
**Option C:** Ganciclovir is an antiviral medication used primarily for cytomegalovirus (CMV) infections, and it is not effective against acyclovir-resistant HSV.
**Clinical Pearl / High-Yield Fact**
Foscarnet is a high-risk medication that requires close monitoring of renal function due to its potential for nephrotoxicity. It is essential to weigh the benefits of foscarnet against the risks of renal impairment, particularly in patients with pre-existing kidney disease.
**Correct Answer:** C. Foscarnet