McArdle disease (Glycogen storage disease type V, GSDV) is an inherited metabolic disorder of skeletal muscle. Affected patients are unable to perform strenuous exercise due to a congenital absence of the enzyme muscle glycogen phosphorylase, essential for glycogen metabolism. This enzyme deficiency results in the inability to mobilise muscle glycogen stores from muscle, required for energy during strenuous exercise. In affected people symptoms of fatigue and cramps occur within minutes of initiating any activity and during strenuous activity such as lifting heavy weights or walking uphill, if activity is continued despite severe cramping, a contracture occurs which leads to muscle damage (rhabdomyolysis), myoglobinuria (dark brown/black discolouration of urine) and, when severe, acute renal failure.