A 32-year-old man from rural Maharashtra presents to the emergency department with a 5-day history of progressive bilateral leg weakness that has now ascended to involve his arms and face. He reports a diarrheal illness 2 weeks prior, treated symptomatically at home. On examination, he has flaccid paralysis of lower limbs with absent deep tendon reflexes, and mild facial weakness. Vital signs show blood pressure 140/90 mmHg, heart rate 102/min with occasional ectopy, and respiratory rate 18/min. Cerebrospinal fluid analysis shows protein 85 mg/dL with normal cell count and glucose. Nerve conduction studies demonstrate prolonged distal latencies and conduction blocks in multiple nerves. What is the most appropriate next immediate management?