A 19-year-old man complains of “brown urine” and pain in the muscles of his arms and legs experienced while playing touch football. He has had several episodes of muscle pain during exercise, but he had not noticed darkening of his urine afterward. The pain usually resolved overnight. Physical examination reveals a well-fed male of normal stature. Reflexes and range of motion in all arms and legs are normal, but there is some paraparesis (weakness), especially in his right leg. A muscle biopsy is taken and sent for specialized resting. The patient is sent home with a recommendation to take a dietary carnitine supplement. Which of the following is the most likely diagnosis?