A 28-year-old woman presents with sudden-onset left eye pain and blurred vision for 3 days. She reports pain on eye movements. On examination, visual acuity is 6/36 in the left eye. Fundoscopy reveals hyperaemia of the optic disc with blurred margins, mild disc swelling, and a central scotoma on visual field testing. Colour vision is markedly impaired (red desaturation). MRI brain and orbits shows T2 hyperintensity in the left optic nerve. Intracranial pressure is normal on lumbar puncture. What is the most likely diagnosis?