A 55-year-old woman is evaluated in the emergency department for altered mental status that has developed over the past week. Her family notes that she has become increasingly confused, and her oral intake has decreased significantly. Her medical history includes breast cancer that was diagnosed 5 years ago. Additional medical history is significant for hypertension. Her only medication is chlorthalidone.
On physical examination, the patient is minimally responsive. Mucous membranes are dry. Temperature is 36.2 °C (97.2 °F), blood pressure is 100/59 mm Hg, pulse rate is 110/min, and respiration rate is 12/min. BMI is 22. There is mild, diffuse abdominal tenderness to palpation. The remainder of the examination is unremarkable.
Laboratory studies are significant for a serum sodium level of 148 mEq/L (148 mmol/L), creatinine level of 4.0 mg/dL (353.6 µmol/L), baseline creatinine level is 1.6 mg/dL (141.4 µmol/L), and calcium level of 18.2 mg/dL (4.6 mmol/L).
Chlorthalidone is discontinued, and intravenous normal saline is started.
Which of the following is the most appropriate immediate next step in management?