A 69-year-old man presents to the emergency depament with painless vision loss of his right eye. He describes the visual loss as a gradual progression from blurry to total blackout over the past two hours. He has no history of prior visual problems. Past medical history is significant for a myocardial infarction three years ago. The patient takes aspirin daily. Vital signs are normal. Physical examination reveals 20/20 vision of the left eye but no vision in the right eye. Extraocular muscles are intact. The neurologic examination is normal. The cardiac examination reveals an S4 hea sound. The most likely cause of this unilateral blindness is which of the following?