A 52-year-old man with poorly controlled HIV infection presents with a painful vesicular rash along the distribution of his left T4 dermatome. PCR of a vesicle swab confirms reactivation of varicella–zoster virus. He is started on an oral agent that is a prodrug requiring initial phosphorylation by a viral kinase and subsequent phosphorylation by host kinases to yield a guanosine analog triphosphate that causes chain termination of viral DNA. Which of the following best describes the mechanism of action of this agent?