Myophosphorylase deficiency (McArdle's disease), or glycogen storage disease type 5 (GSD5), is a severe form of glycogen storage disease characterized by exercise intolerance. The disease is Monarch Disease Ontology id MONDO_0009293 (glycogen storage disease V). Also known as: GSD due to muscle glycogen phosphorylase deficiency, GSD type 5, GSD type V, McArdle disease, McArdle's disease, Mcardle disease, PYGM glycogen storage disease, glycogen storage disease caused by mutation in PYGM.