Acyclovir is a nucleoside analogue used for herpes simplex and varicella-zoster infections. For acyclovir to become active, it must first be monophosphorylated by a virus-encoded enzyme. Which enzyme performs this initial activation step?

• A Host cell thymidine kinase
• B Host cell DNA polymerase
• C Viral DNA polymerase
• D Viral thymidine kinase ✓

Explanation

Acyclovir requires initial monophosphorylation by viral thymidine kinase in herpes-infected cells. Host kinases then convert it to the active triphosphate form, which competitively inhibits viral DNA polymerase and causes chain termination. This viral enzyme dependency explains acyclovir selectivity. Host thymidine kinase does not efficiently phosphorylate acyclovir, which is why it spares uninfected cells.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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