The presentation, exercise induced cramps and myalgia (including chest pain) as well as exercise intolerance , and has  two  episodes of rhabdomyolysis; is highly suggestive of a metabolic myopathy. Specifically, glycogen storage diseases due to deficiency of myophosphorylase (McArdle’s), phosphofructokinase, and phosphoglycerate mutase are to be considered. The best guess is McArdle’s disease, given the description of “second wind phenomenon”. Another important cause of recurrent rhabdomyolysis is deficiency of carnitine palmitoyl transferase type 2 (CPT-2, a lipid pathway enzyme), but CPT-2 is characterized by symptoms (including rhabdomyolysis) which occur after prolonged exercise or fasting. Other differential diagnoses include mitochondrial myopathy and dystrophinopathy (Becker muscular dystrophy). However the latter two conditions are usually associated with fixed muscle weakness.

What is the next step? Diagnosis? (click here)