A 24-year-old man comes to his doctor because of a 3-month history of pain in the arms and legs after strenuous exercise. He also had severe cramps and passed red urine after digging out several bushes from his front yard. Physical examination now shows abnormalities. This patient most likely has an inherited deficiency in which of the following enzymes?

Question

Quizplus · Accepted Answer

The symptoms described, such as muscle pain and cramps after exercise and passing red urine (myoglobinuria), are indicative of McArdle's disease, which is caused by a deficiency in muscle phosphorylase.  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 24-Year-Old Man Comes to His Doctor Because of a 3-Month