A 25-year-old woman complains of weakness and easy fatigability, which is most pronounced in the late afternoon. She describes difficulty reading and tiredness while watching television. She has problems chewing and swallowing and loses her voice while talking. Physical examination reveals ptosis and diplopia. Laboratory studies would most likely demonstrate serum autoantibodies directed against which of the following proteins?

Acetylcholine receptor. Myasthenia gravis is an acquired autoimmune disease characterized by abnormal muscular fatigability. It is caused by circulating antibodies to the acetylcholine receptor at the myoneural junction (motor endplate). Antibodies to the acetylcholine receptor can be demonstrated in the serum of most patients with myasthenia gravis and localized in muscle biopsies by immunohistochemistry. The clinical severity of the condition is variable, and symptoms tend to wax and wane. The other choices are not related to myasthenia gravis.Diagnosis: Myasthenia gravis