A 58-year-old man from Delhi presents with sudden-onset painless vision loss in the left eye for 2 days. He reports seeing a dark shadow that started from the superior temporal field and progressively moved downward. On examination, visual acuity is 6/9 in the left eye. Dilated fundoscopy reveals a pale, edematous optic disc with blurred margins and retinal whitening in the distribution of the superior branch of the central retinal artery. The right eye is normal. What is the most likely diagnosis?