A 45-year-old woman presents with perioral paresthesia and carpopedal spasm 2 days after thyroidectomy for Graves' disease. An ECG is performed and shows a prolonged QT interval. The structure marked **B** in the diagram (ST segment lengthening) is the primary ECG abnormality responsible for QT prolongation in this patient's condition. Which of the following is the MOST likely underlying cause of her ECG findings?