Which explanation best accounts for the selective activity of acyclovir against herpes viruses but not most host cells?

• A. Acyclovir must be converted into a protein by viral proteases which then embeds in the viral envelope preventing the virus from entering neighboring cells; therefore it only acts where viral proteases are present.
• B. Acyclovir is preferentially phosphorylated by the viral thymidine kinase encoded by herpes, producing the active triphosphate that viral DNA polymerase incorporates, causing chain termination.
• C. Acyclovir passively diffuses only into infected cells because herpes virions alter membrane lipid composition so the drug is trapped solely within infected cells and excluded from uninfected host cells.
• D. Acyclovir binds directly to host topoisomerase II only when viral proteins are present, creating DNA breaks that exclusively affect viral genomes while leaving host chromosomes intact.

Answer: (B)