Neuromuscular Disorders USMLE Step 1 Practice Question

A 49-year-old man presents with 3 months of progressive symmetric leg weakness and paresthesias. He reports difficulty climbing stairs and denies bowel/bladder dysfunction. Vital signs: BP 138/82, HR 88, RR 16, Temp 37°C, SpO2 98% on room air. Examination reveals absent deep tendon reflexes bilaterally. Nerve conduction studies demonstrate segmental demyelination with slowed conduction velocities. CSF protein is elevated at 78 mg/dL. He improves significantly after corticosteroid therapy. Which of the following is the most likely diagnosis?