Active 46-year-old man with right-sided visual loss and no family history of stroke · Dx?

A 46-year-old man presented to the emergency department with sudden-onset right-sided visual loss. He had a history of asthma, but no family history of hypercoagulability, deep vein thrombosis, or stroke. The patient had an active lifestyle that included scuba diving, mountain biking, and hockey (coaching and playing). The physical examination revealed a right homonymous upper quadrantanopia. The neurologic examination was within normal limits, except for the visual deficit and unequal pupil size. A computerized tomography scan of the patient's head did not reveal any lesions.