A 32-year-old woman presents with sudden-onset blurred vision in the left eye for 3 days, associated with pain on eye movements. On examination, visual acuity is 6/36 in the left eye, and the left optic disc appears hyperaemic with blurred margins. Fundoscopy shows flame-shaped haemorrhages around the disc. Which investigation is most appropriate to confirm the diagnosis of optic neuritis and exclude papilloedema?