A 64-year-old man is evaluated because of weakness and difficulty in weaning from mechanical ventilation. The patient had been admitted to the intensive care unit 2 weeks ago because of septic shock related to alcoholism, pneumonia, and Klebsiella bacteremia. He had developed respiratory failure requiring intubation and mechanical ventilation as well as acute kidney injury. His pulmonary infiltrates had responded to appropriate intravenous antibiotics and his hypotension had responded to intravenous norepinephrine. Now the patient is alert and responsive to verbal commands, is afebrile with blood pressure of 114/74 but has not tolerated several trials of weaning from the ventilator. On physical examination, the patient is cooperative. Cranial nerves are normal. Muscle strength is poor, especially in distal musculature, where he displays only 2/5 strength in the hands and feet. Proximal strength is 3/5. Ankle and knee reflexes are unobtainable. Sensory examination is difficult because of problems communicating with the patient but suggests distal sensory loss in the lower extremities. Laboratory studies show that his creatinine level has spontaneously improved to 2.4 mg/dL. Electrolytes are normal, and the patient has a mild normochromic normocytic anemia with resolving leukocytosis. Serum creatine kinase is 78 units/L (normal <140). What is the most likely cause of his weakness?