Acyclovir exerts selective toxicity against herpes simplex virus because the first phosphorylation step inside an infected cell is carried out by:

• A The viral DNA polymerase
• B Host cell thymidylate kinase
• C The viral thymidine kinase ✓
• D The virion-associated ribonucleotide reductase

Explanation

HSV encodes its own thymidine kinase, which phosphorylates acyclovir to the monophosphate with far greater efficiency than host kinases. Host enzymes then convert it to the triphosphate, which selectively inhibits viral DNA polymerase and causes chain termination because it lacks the 3'-hydroxyl group. This dependence on viral thymidine kinase explains both selectivity and the common mechanism of acyclovir resistance, namely loss or alteration of that enzyme.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.