A 16-year-old female presents with intermittent muscle cramps and fatigue. Her laboratory workup is notable for a serum potassium of 2.8 mEq/L, serum bicarbonate of 29 mEq/L, and serum magnesium of 1.3 mg/dL. A 24-hour urine collection reveals low calcium excretion and elevated magnesium excretion. Which genetic defect is the most likely cause of her condition?