A 27-year-old woman presents with acute vision loss and pain in the left eye, but no other symptoms. On examination, she appears well, visual acuity is not possible, and she can only perceive movement and bright light. The direct pupillary reflex is absent but the indirect (consensual) response is normal. The optic disc is edematous. An example of the pathologic changes seen in her optic nerve are shown in Figure A. It shows a plaque of demyelination in the optic nerve as compared to a normal sample in Figure B. Which of the following is the most likely diagnosis?Figure AFigure B