Which statement best describes the role of Thymidine kinase in viral infections treated with acyclovir?

Thymidine kinase (TK) plays a crucial role in the action of acyclovir against viral infections, particularly those caused by herpes simplex virus (HSV) and varicella-zoster virus (VZV). Acyclovir is a nucleoside analog that requires phosphorylation to be activated, and that activation primarily happens through the action of viral thymidine kinase.

When acyclovir enters an infected cell, TK phosphorylates it into acyclovir monophosphate. This monophosphate form is then further phosphorylated by cellular kinases to form acyclovir triphosphate, which is the active form that inhibits viral DNA polymerase and incorporates into viral DNA, leading to chain termination during replication.

The role of thymidine kinase in this context emphasizes its ability to selectively activate acyclovir in infected cells, where the enzyme is produced, because uninfected cells typically lack significant levels of this viral enzyme. This selectivity contributes to the therapeutic efficacy of acyclovir while minimizing toxicity to host cells.