A 72-year-old woman with hypertension on lisinopril and a recent diagnosis of acute kidney injury (serum creatinine 2.4 mg/dL, baseline 0.9 mg/dL) is found to have serum potassium 6.8 mEq/L on routine blood work. She is asymptomatic, and ECG shows no acute changes. Urine output is 800 mL/day. What is the most appropriate next step in management?