The patient was a 64-year-old man who complained of a gradual loss of vision in the left eye over the last two weeks. This vision was 20/20 OD and 20/25 OS. There was an afferent pupillary defect and an inferior visual field defect in the left eye. There was red desaturation in the left eye, and a bright light appeared to be twice as bright in the right eye compared to the left eye. He denied other neurological symptoms, and he denied headaches or other common symptoms of giant cell arteritis. His examination was remarkable for disc edema in the left eye and an elevated disc in the right eye with a sharp margin. The anterior and posterior chambers were quiet. Six weeks later, he described a further loss of vision in the left eye, and the acuity was 20/40. The disc edema had resolved and there was disc pallor. Three months later, the clinical presentation and vision were unchanged.
Laboratory testing revealed an unremarkable CBC, normal C-reactive protein, angiotensin converting enzyme, fasting glucose, and a sed rate of 1. An MRI of the brain and orbits was unremarkable. What is the most likely diagnosis?