A 45-year-old woman with Crohn's disease and a small intestinal fistula develops tetany during the 2nd week of parenteral nutrition. The laboratory findings includeCa: 8.2 mEq/LNa: 135 mEq/LK: 3.2 mEq/LCl: 103 mEq/LPO4: 2.4 mEq/LAlbumin: 2.4pH: 7.4838 kPaP: 84 kPa bicarbonate 25 mEq/LThe most likely cause of the patient's tetany is