During the past 3 years, a 20-year-old man has had increasing fatigue and muscle pain in his legs after jogging. Following exercise of his left upper arm, the lactate concentration remains decreased in blood obtained from a vein draining the exercised muscle. A deficiency is most likely in which of the following enzyme activities in the muscle?

Question

Quizplus · Accepted Answer

The symptoms and exercise intolerance suggest McArdle's disease, which is caused by a deficiency in muscle glycogen phosphorylase. This enzyme is crucial for glycogen breakdown, and its deficiency leads to decreased lactate production during exercise.  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.

During the Past 3 Years, a 20-Year-Old Man Has Had