Glycogen storage disease type V

 A 20-year-old man comes to the office for the evaluation of exercise intolerance. The patient mentions that he recently started a summer job as football coach assistant and that he is concerned because he can't keep up with the physical demand of his new job. Upon further interrogation, the patient says that he has severe and painful cramping on the first 15 to 20 minutes of warm-up exercise, and after this period he can exercise without a problem. Physical examination shows minimal proximal weakness. His temperature is 36.7°C (98°F), pulse is 110/min, respirations are 14/min, blood pressure is 120/80 mmHg, and saturation is 82%. Fasting blood glucose is 87 mg/dL (4.8 mmol). Urinalysis after exercise is obtained and shown below.  


A deficiency of which of the following enzyme activities is the most likely cause of the findings in this patient?