A 9-year-old boy presents with a several day history of progressive arm and leg weakness. He has been well except for an upper respiratory infection two weeks ago. The patient is ale and oriented. On repeated examination, the hea rate varies between 60 and 140 beats/minute and the blood pressure varies between 90/60 and 140/90 mm Hg. Respirations are shallow with a rate of 50/minute. There is symmetric weakness of the face and all four extremities. Deep tendon reflexes are absent. Sensation is intact. The most likely diagnosis is: