A 22-year-old woman presents with diplopia, the inability to smile or frown, and difficulty in walking—all since waking up six hours ago. Careful questioning reveals a mild diarrheal episode two weeks ago, which resolved spontaneously. Her surgical and family histories are unremarkable. She does not smoke, only drinks socially, and denies using recreational drugs. She is sexually active, with a single stable partner. There is no history of recent travel or recent outdoor activities.