A 72-year-old man with chronic kidney disease (baseline creatinine 2.5 mg/dL, eGFR 22 mL/min/1.73m²) is admitted with acute decompensated heart failure. He is started on intravenous furosemide 80 mg twice daily. On day 3, his serum creatinine rises to 3.8 mg/dL, urine output is 0.5 mL/kg/hr, and serum potassium is 6.2 mEq/L. Urinalysis shows no casts or proteinuria. Renal ultrasound is normal with no obstruction. What is the most appropriate immediate next step in management?