A 72-year-old man with atrial fibrillation notices sudden, profound loss of vision in his right eye while eating breakfast. There is no pain, headache or preceding visual disturbance. The right pupil responds poorly when light is shone into that eye, and fundoscopy shows a pale retina with markedly narrowed retinal arteries; the foveal area remains conspicuously red. Which one of the following is the most likely diagnosis?