You are answering a USMLE-style medical question. Question: A 37-year-old woman with an HIV infection comes to the physician for a follow-up examination. Six months ago, combined antiretroviral therapy consisting of dolutegravir, tenofovir, and emtricitabine was initiated. Laboratory studies show a decrease in the CD4 count and an increase in the viral load despite ongoing treatment. The patient is switched to a new treatment regimen, including a drug that acts by preventing viral DNA synthesis without undergoing intracellular phosphorylation. Which of the following is the most likely drug? Options: A) Efavirenz B) Ritonavir C) Raltegravir D) Lamivudine " Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.