PYGM and glycogen storage disease V: McArdle’s disease, known as glycogen storage disease V, arises from mutations in the PYGM gene encoding muscle glycogen phosphorylase (myophosphorylase), resulting in exercise intolerance including fatigue, muscle soreness, and cramps during the initial stages of exercise.[1] The onset of symptoms typically occurs during childhood, with approximately 60% in the first decade and 28% in the second decade of life.