38 year old man was evaluated in the emergency department because of sudden onset of chest pain after heavy exertion of his arms (trying to move a dock) e CPK was 5000 U/L and cardiogenic causes were ruled out (normal CK MB and troponin). He mentioned difficulty running since childhood because of muscle cramps and pain, but he could resume running after a brief period of rest. There is H/O two episodes of “dark urine” after strenuous exercise for which he did not seek medical attention. Physical and neurological exams were unremarkable including normal muscle strength and DTRs. Two months after being discharged from the hospital, CK was 1200 U/L and nerve conduction study and EMG were normal.

What is the differential diagnosis and next workup? (click here)