Q. A 12-year-old boy is brought to the clinic by his concerned parents. They recount that during his basketball training sessions, he consistently starts with severe fatigue, muscle pain, and cramps. However, about 10 minutes into the exercise, his performance improves noticeably, and he's able to continue with less difficulty. The boy also describes an unusual swelling of his muscles after intense activity. His parents are particularly alarmed by a recurring dark discoloration of his urine, especially after rigorous training sessions. Recent laboratory tests reveal a marked elevation in a specific muscle enzyme. Based on the clinical findings and laboratory reports, what is the most likely diagnosis?