A 72-year-old woman with newly diagnosed multiple myeloma presents with confusion, constipation and weakness. Her serum calcium is 4.18 mmol/L (2.10–2.60), creatinine is 420 micromol/L (45–90), and she is producing less than 10 mL of urine per hour. A cautious saline infusion has caused pulmonary oedema, and her ECG shows a shortened QT interval. Which one of the following is the most appropriate next treatment?