A 70-year-old male develops ascending motor weakness and hyporeflexia one week after an influenza vaccination. No fever. What is the most likely diagnosis?

Guillain-Barre syndrome (GBS): ascending flaccid paralysis with areflexia, classically 2-4 weeks post-viral infection or vaccination (including influenza). Post-infectious autoimmune demyelinating polyneuropathy. ALS = progressive motor neuron disease not triggered by vaccination. MS = CNS demyelination (relapsing-remitting).