A 24-year-old male presents to the emergency department with dark red-brown urine and severe myalgia following an intense gym session. He reports a lifelong history of exercise intolerance and painful muscle cramps that resolve after 10–15 minutes of rest, after which he can resume activity. His serum creatinine is elevated at 2.8 mg/dL. An ischemic forearm exercise test shows **failure of venous lactate to rise above baseline**, despite normal ammonia elevation. The structure marked **C** in the diagram is deficient in this patient. Which of the following best explains the pathophysiology of his presentation?