Demyelinating Diseases USMLE Step 1 Practice Question

A 19-year-old boy develops acute ascending weakness in all four limbs and dyspnea over 24 hours following a viral upper respiratory infection. Vital signs: BP 118/76, HR 102, RR 22, temp 37.2°C, SpO2 98% on room air. Lumbar puncture reveals elevated protein (78 mg/dL) with normal glucose (52 mg/dL). EMG demonstrates demyelinating slowing without conduction blocks. Cranial nerves remain intact. Which diagnosis best explains these findings?