A 70-year-old male received the influenza vaccine one week ago. He now presents with progressive ascending motor weakness but no fever. What is the most likely diagnosis?

Guillain-Barré Syndrome (GBS) is an acute autoimmune polyneuropathy characterized by ascending motor weakness with areflexia, often triggered by infection or vaccination (notably influenza). The temporal relationship (1–3 weeks post-vaccination or infection), absence of fever, and progressive weakness are classic. ALS is a progressive motor neuron disease (UMN + LMN) not triggered acutely by vaccines. GBS treatment includes IVIG or plasmapheresis.