Demyelinating Diseases USMLE Step 1 Practice Question

A 28-year-old man presents with acute bilateral leg weakness and sensory loss below the T8 dermatome following a viral illness two weeks prior. Vital signs: BP 128/82, HR 92, RR 18, Temp 37.2°C, SpO2 98% on room air. MRI cervical spine demonstrates extensive T2 hyperintensity spanning three vertebral levels. CSF analysis reveals elevated protein (78 mg/dL) and pleocytosis (45 WBC/μL). Cranial nerves remain intact. Which diagnosis is most likely?