A 35-year-old HIV-positive man in Mozambique develops dysphagia and substernal pain. He has oral thrush. His CD4 count is 45 cells/µL. He is treated empirically with fluconazole for 14 days but shows no improvement. Upper GI endoscopy reveals large, deep, solitary oesophageal ulcers with a punched-out appearance. Biopsy shows intranuclear inclusion bodies (owl-eye cells). What is the most likely diagnosis?