A 68-year-old man with newly diagnosed HIV (CD4 count 35 cells/μL) presents with sudden vision loss and floaters. Fundoscopy reveals hemorrhagic retinal infiltrates with granular appearance. A retinal biopsy is performed, and histopathology shows enlarged cells with large basophilic intranuclear inclusions surrounded by a clear halo, as marked **A** in the diagram. Which of the following is the most appropriate first-line antiviral therapy for this patient?