A 15 year old female patient presents with pain in the calf muscles on exercise. Routine investigation reveals burgundy-colored urine (sample obtained after exercise). Histopathology of muscle shows deposits of glycogen in an excess amount. What is the diagnosis?

Type V Glycogen Storage Disease (McArdle Disease) Type V GSD is an autosomal recessive disorder. Biochemical Defect: Muscle Phosphorylase Defect. The gene for muscle phosphorylase (PYGM) has been mapped to chromosome 11q13. Lack of this enzyme limits muscle ATP generation by  glycogenolysis, resulting in muscle glycogen accumulation, and is the prototype of muscle energy disorders. Clinical Manifestations: Symptoms usually 1st develop in late childhood or as an adult and are characterized by exercise intolerance with muscle cramps and pain.  Many patients experience a characteristic ‘second wind’ phenomenon. If  they slow down or pause briefly at the 1st appearance of muscle pain, they can resume exercise with more ease. About 50% of patients report burgundy-colored urine after exercise, which is the consequence of exercise-induced myoglobinuria, secondary to rhabdomyolysis. Key Concept: Burgundy-colored urine after exercise is the consequence of exercise-induced myoglobinuria, secondary to rhabdomyolysis. It occurs in Type V Glycogen Storage Disease (McArdle Disease). Reference- Harper’s illustrated biochemistry. 30th edition page no: 179