Aciclovir Mechanism — MFDS Part 1 MCQ
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Correct answer: A — Viral thymidine kinase initiates phosphorylation, cellular kinases form the triphosphate, and viral DNA polymerase is inhibited with DNA-chain termination
Explanation lettering: C = shown as B · E = shown as C · B = shown as D · D = shown as E
A is correct. Aciclovir is a purine nucleoside analogue whose selective activation begins when herpesvirus-encoded thymidine kinase converts it to aciclovir monophosphate. Cellular enzymes subsequently produce the active triphosphate. Aciclovir triphosphate preferentially inhibits viral DNA polymerase and is incorporated into viral DNA, resulting in chain termination. Normal cellular thymidine kinase uses aciclovir poorly, contributing to selectivity; therefore B reverses the relevant viral and cellular enzyme roles and incorrectly targets host DNA polymerase. Aciclovir does not inhibit viral protease-mediated protein processing, so C is incorrect. Herpes simplex is a DNA virus, and aciclovir does not target RNA-dependent RNA polymerase, excluding D. It acts after cellular entry rather than blocking viral attachment or entry, excluding E.
Reference: Electronic Medicines Compendium. Aciclovir 200 mg Tablets, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties. Updated 2026. https://www.medicines.org.uk/emc/product/11673/smpc