A 20-year-old woman presents with a history of rapid loss of vision in one eye. Examination reveals pain on movement of the eyeball. The appearance of the fundi is normal, but the afferent pupillary response is diminished. Perimetry shows a large central scotoma. Which of the following is the most likely diagnosis?

In the vast majority of cases, retrobulbar neuritis occurs as an episode in a demyelinating disease such as multiple sclerosis. It is the first manifestation of multiple sclerosis in 15% of cases and occurs at some point in 50% of all patients with the disease. The course of the retrobulbar neuritis is that of gradual spontaneous improvement.