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

Magnesium deficiency is common in malnourished patients and patients with large gastrointestinal fluid losses. The neuromuscular effects resemble those of calcium deficiency-namely, paresthesia, hyperreflexia, muscle spasm, and ultimately tetany. The cardiac effects are more like those of hypercalcemia. An electrocardiogram therefore provides a rapid means of differentiating between hypocalcemia and hypomagnesemia. Hypomagnesemia also causes potassium wasting by the kidney. Many hospital patients with refractory hypocalcemia will be found to be magnesium deficient. Often this deficiency becomes manifest during the response to parenteral nutrition when normal cellular ionic gradients are restored. A normal blood pH and arterial PCO2 rule out hyperventilation. The serum calcium in this patient is normal when adjusted for the low albumin. Hypomagnesemia causes functional hypoparathyroidism, which can lower serum calcium and thus result in a combined defect.