A 28-year-old woman with a history of recurrent myoglobinuria and exercise intolerance is found to have a deficiency of phosphofructokinase (PFK), the rate-limiting enzyme of glycolysis. She presents with severe muscle cramps and dark urine after a 2-hour gym session. Her serum creatinine is 1.2 mg/dL and urine myoglobin is positive. What is the most appropriate immediate management step?