A 52-year-old woman with a history of diabetes mellitus type 2 presents with painless vision loss in the left eye over 3 weeks. On examination, the left pupil is 3 mm and reacts sluggishly to light compared to the right (4 mm, brisk reaction). There is a relative afferent pupillary defect (RAPD) in the left eye. Fundoscopy shows optic disc pallor and flame-shaped hemorrhages in the left eye. What is the most appropriate investigation to confirm the underlying pathology?