A 35-year-old woman from Mumbai presents with progressive vision loss in the left eye over 3 weeks and headaches. On examination, the left pupil is 4 mm and reacts sluggishly to direct light, but briskly to consensual stimulation. The right pupil is 3 mm and reacts normally. Fundoscopy of the left eye shows disc pallor and a few scattered cotton-wool spots. Dilated fundus examination of the right eye is normal. What is the most likely diagnosis?