Demyelinating Diseases USMLE Step 1 Practice Question

A 19-year-old man presents with acute paraplegia and loss of pain/temperature sensation at the T10 level; vibration sense is preserved. Vital signs: BP 118/76, HR 88, RR 16, Temp 37.2°C, SpO2 98% on room air. Spinal MRI demonstrates extensive T2 hyperintensity spanning T4-T12 with central cord predominance. CSF analysis shows mild lymphocytic pleocytosis. He denies fever, rash, or recent vaccinations. He takes no medications. Which diagnosis best explains these findings?