A previously healthy patient received an influenza vaccination one week ago and now presents with rapidly ascending symmetrical muscle weakness that started in the legs. What is the most likely diagnosis?

Correct answer: D. Guillain-Barré syndrome

• A. Incorrect. ALS is a progressive degenerative motor neuron disease with mixed upper and lower motor neuron signs developing over months to years, not over days.
• B. Incorrect. MS produces relapsing, ASYMMETRICAL, disseminated CNS deficits (optic neuritis, internuclear ophthalmoplegia, sensory levels) over years — not an acute ascending symmetrical weakness after a vaccine.
• C. Incorrect. Myasthenia gravis causes FATIGABLE weakness that is worse with use and better with rest, characteristically affecting ocular and bulbar muscles first (ptosis, diplopia, dysphagia) — a descending, fluctuating pattern.
• D. Correct. GUILLAIN-BARRÉ SYNDROME is an acute immune-mediated demyelinating polyradiculoneuropathy that classically follows an antecedent infection (Campylobacter jejuni, CMV, EBV) or, less commonly, vaccination by 1–3 weeks. The hallmark is ASCENDING, SYMMETRICAL flaccid weakness with AREFLEXIA, progressing over days. The danger is respiratory muscle involvement, so these patients need urgent hospital assessment.