A 28-year-old woman presents with acute loss of vision in the left eye over 24 hours. She reports pain on eye movement and has a history of multiple sclerosis diagnosed 2 years ago. On examination, visual acuity is 6/60 in the left eye with a relative afferent pupillary defect (RAPD). Fundoscopy shows hyperaemia of the optic disc with blurred margins, mild disc swelling, and peripapillary flame-shaped haemorrhages. Visual evoked potentials (VEP) show delayed latency. What is the most likely diagnosis?