An AIDS patient on suppressive acyclovir therapy develops progressive mucocutaneous herpetic ulcers despite dose escalation. The isolate shows absent thymidine kinase activity. Which drug should be used next?

• A Valacyclovir at higher dose
• B Ganciclovir
• C Famciclovir
• D Foscarnet ✓

Explanation

Acyclovir, valacyclovir and famciclovir all require viral thymidine kinase for initial phosphorylation, so TK-deficient mutants are cross-resistant to the entire class. Foscarnet inhibits the viral DNA polymerase directly without needing TK activation, making it the drug of choice for acyclovir-resistant herpesvirus disease. Ganciclovir also depends partly on viral kinase phosphorylation and is not indicated for resistant HSV.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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