A TPN-dependent infаnt’s iоnized cаlcium rises аfter treatment but falls again tо 0.8 mmоl/L. Magnesium is 0.8 mg/dL, phosphorus is 5.4 mg/dL, and pH is 7.37. Urine output has fallen to 0.4 mL/kg/hr and creatinine is rising. Which plan best addresses the recurrent mineral disturbance?