An 18-year-old man presents with sudden profound generalized weakness upon waking up in the morning. The previous night, the patient had worked out in the gym and then went out with his friends for dinner. Nothing significant is noted on history except he has had similar episodes in the past after exercise and large meals. Which of the following tests would be most useful in diagnosing this patient?

A. acetylcholine receptor antibodies
B. antinuclear antibody
C. serum calcium
D. serum potassium
E. serum thyroxine