A 12-year-old girl from Delhi presents with progressive muscle weakness, myalgia, and dark-colored urine after strenuous exercise. Serum creatine kinase is 8,500 U/L, myoglobin is elevated, and urine myoglobin is positive. Her mother reports a similar episode 2 years ago. Muscle biopsy shows excess glycogen with normal structure. Phosphorylase activity is normal. What is the most appropriate next step in management?