A 20-year-old man comes to the physician because of progressive, painful muscle cramps and intolerance to exercise during the past 3 months. On one occasion, he passed burgundy-colored urine. His serum lactate concentration does not increase in blood from the vein proximal to an exercised right forearm. A biopsy specimen of muscle shows an increased concentration of glycogen. Which of the following enzymes is most likely deficient in this patient's muscle?

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The symptoms and findings are consistent with McArdle's disease, which is caused by a deficiency of muscle glycogen phosphorylase. This leads to an inability to break down glycogen in muscle, resulting in exercise intolerance, muscle cramps, and myoglobinuria.  Deficiency in muscle phosphorylase activity is associated with the development of McArdle disease (glycogen storage disease type V [GSDV]). The clinical presentation of GSDV is usually seen in young adulthood and is characterized by exercise intolerance and muscle cramps following slight exercise. Attacks of myoglobinuria frequently accompany the muscle symptoms of GSDV. About half of GSDV patients will exhibit burgundy-colored urine after exercise.

A 20-Year-Old Man Comes to the Physician Because of Progressive