A 45-year-old woman with Crohn disease and a small intestinal fistula develops tetany during the second week of parenteral nutrition. The laboratory findings include Na: 135 mEq/L K: 3.2 mEq/L Cl: 103 mEq/L HCO3 : 25 mEq/L Ca: 8.2 mEq/L Mg: 1.2 mEq/L PO4 : 2.4 mEq/L Albumin: 2.4 An aerial blood gas sample reveals a pH of 7.42, PCO2 of 38 mm Hg, and PO2 of 84 mm Hg. Which of the following is the most likely cause of the patient's tetany?