A 19-year-old man has early fatigue and muscle cramps while playing sports. He is fine when walking or doing light levels of work. On examination, he appears well and the muscle strength in the proximal muscles is normal. There is no muscle fatigue with repetitive arm grip exercises. After an exercise stress test, his serum creatine kinase (CK) is elevated and lactate level is normal. Which of the following is the most likely diagnosis?