A previously healthy patient received an influenza vaccination one week ago and now presents with rapidly progressive muscle weakness. What is the most likely diagnosis?

Correct answer: C. Guillain-Barré syndrome

• A. Incorrect. ALS is a slowly progressive degenerative motor neuron disease with mixed upper and lower motor neuron signs.
• B. Incorrect. Myasthenia gravis causes fatigable weakness that worsens with use, classically with ptosis and diplopia, and is not vaccine-triggered.
• C. Correct. Guillain-Barré is an acute immune-mediated demyelinating polyneuropathy classically triggered 1–3 weeks after an infection or vaccination, producing rapidly ascending symmetric weakness.
• D. Incorrect. MS presents with relapsing-remitting focal neurologic deficits over months to years, not acute post-vaccination ascending weakness.