<s>[INST] context: This patient has a sudden onset of complete, unilateral, painless loss of vision with intact extraocular muscle movement. This is consistent with occlusion of the retinal aery. The retina derives its blood supply from the internal carotid aery the ophthalmic aery, from which branch the central retinal aery and ciliary aeries. The central retinal aery supplies the inner layers of retina, while ciliary aeries supply blood to the choriocapillaris and the outer retinal layers. Occlusion of small retinal aerioles produces "cotton wool" spots while total occlusion of the ophthalmic aery leads to total ischemia and blindness. This usually occurs secondary to atherosclerotic disease. The key here is recognizing that the loss of vision is an isolated finding with no other neurologic symptoms. Other deficits would be consistent with a stroke of a larger vascular territory such as the internal carotid aery. Carotid aery dissection is less likely than retinal aery occlusion because there would likely be multiple associated neurologic findings in the case of dissection, as the internal carotid aery supplies a large poion of the anterior circulation of the brain. The most likely symptoms among a large cadre of possibilities is face and arm weakness contralateral to the dissection. Cavernous sinus thrombosis would lead to facial pain and possible cranial nerve deficits. The optic nerve (cranial nerve II) extends anteriorly from the optic chiasm through the superior orbital fissure. Posterior cerebral aery occlusion is less likely than retinal aery occlusion because there would likely be multiple associated neurologic findings. Also, the visual field loss resulting from a unilateral posterior cerebral aery occlusion would more likely be a homonymous hemianopia, it would not cause complete blindness in one eye. Other changes that may occur from posterior cerebral aery stroke are complex findings, such as the inability of recognize faces (prosopagnosia), failure to connect visual perception with conscious thought, color vision loss, and difficulty with reading (dyslexia or alexia). Memory problems and motor impairment are also common associated findings. [/INST] question: 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? | options: A: Carotid aery dissection B: Cavernous sinus thrombosis C: Posterior cerebral aery occlusion D: Retinal aery occlusion | answer: D </s>