A 19-year-old man comes to the physician because of muscle cramping that began after he participated in a race 24 hours ago. He has had similar but mild symptoms since adolescence that have progressed in severity since he joined a college track team. Physical examination shows no abnormalities. No significant lactate concentration increase is observed in blood obtained distal to an exercised ischemic right arm. A deficiency of which of the following in muscle is the most likely cause of this patient's symptoms?

Question

Quizplus · Accepted Answer

The symptoms and lack of lactate increase suggest McArdle's disease, which is due to a deficiency of muscle glycogen 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 19-Year-Old Man Comes to the Physician Because of Muscle