Vascular Disease USMLE Step 1 Practice Question

A 68-year-old man with a 12-year history of hypertension and type 2 diabetes mellitus presents to the emergency department with acute painless vision loss in his right eye that began 2 hours ago. He reports the vision loss was instantaneous and describes a dark curtain descending from above. He denies eye pain, flashing lights, or floaters. Vital signs are BP 168/96 mmHg, HR 88/min, RR 16/min, temperature 37.0°C, and SpO2 98% on room air. Current medications include lisinopril and metformin. Visual acuity is count fingers in the right eye. Dilated fundoscopic examination of the right eye reveals retinal whitening and edema in a wedge-shaped distribution corresponding to the superior arcade, with a cherry-red spot noted at the macula. The left eye appears normal. Carotid duplex ultrasonography shows no significant stenosis bilaterally. Which of the following is the most likely diagnosis?