A 70-year-old man on ICU has been ventilated and sedated for 14 days. After extubation, he is noted to have profound symmetrical weakness of all four limbs (MRC grade 2/5 throughout) with preserved sensation. Creatine kinase is normal. Nerve conduction studies show reduced compound muscle action potentials (CMAPs) with normal sensory nerve action potentials (SNAPs) and normal conduction velocities. What is the most likely diagnosis?