A 19-year-old man presents to the clinic complaining of early fatigue and muscle cramps while playing sports. He is fine when walking or doing less intense levels of work. On physical examination, he appears well and the vital signs are normal. Muscle bulk, tone and strength in the proximal muscles are 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?