Glycogen storage disease type 5 (McArdle disease) is an autosomal recessive disease affecting muscle function. A mutation on the muscle glycogen phosphorylase gene, PYGM, creates a myophosphorylase deficiency so that muscle glycogen cannot be adequately broken down. Patients typically present with fatigue, myalgia, muscle cramps or weakness, poor endurance, myoglobinuria, and elevated levels of creatinine kinase. Diagnosis typically occurs by early adulthood but may first present in older adults with progressively worsening weakness. Some patients will experience complications such as rhabdomyolysis or acute renal failure.