Glycogen storage disease type V (GSD V) is an inherited disorder of glycogen metabolism that affects the skeletal muscle. The disease is characterized by muscle fatigue, stiffness, myalgia, and weakness often caused by activity and improved by rest. If activity is continued after symptoms appear, severe, painful muscle cramping and contracture may occur; this may be accompanied by myoglobinuria and can lead to rhabdomyolysis with possible renal failure if not treated properly. Onset of symptoms is usually in early childhood; however, individuals are often not diagnosed until after age 30. One paper reports two individuals diagnosed in their 70s due to very late-onset myopathy with no previous history of exercise intolerance. A subset of patients have fixed muscle weakness and wasting with age. Serum creatine kinase (CK) is generally elevated. The potential risk of statins in individuals with McArdle disease is still under investigation; however several case reports have suggested that statin use may cause an adverse muscle reaction in these patients or uncover the underlying muscle disease. In one U.S. study, the incidence of GSD V was estimated to be about 1 in 100,000.