Case 21: The Stiff Smile

A 29-year-old woman presents with complaints of drooping eyelids and double vision that worsen as the day progresses. She also reports difficulty chewing and swallowing food by evening, though these symptoms improve after rest. On examination, she has bilateral ptosis and weakness in extraocular muscles without sensory deficits. Her deep tendon reflexes are intact. What is the most likely diagnosis, and what is the underlying pathophysiological mechanism causing her symptoms?