A 22-year-old woman presents with the inability to smile or frown since waking up around 6 hours ago. She has difficulty in moving her eyes, complains of double vision, and finds it difficult to walk.

Close questioning reveals a mild diarrheal episode about 2 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.