A 42-year-old woman with systemic lupus erythematosus on mycophenolate mofetil presents with fatigue and muscle cramps. Laboratory findings: serum potassium 6.8 mEq/L, eGFR 35 mL/min/1.73m², serum bicarbonate 18 mEq/L, and serum creatinine 1.9 mg/dL. ECG shows normal sinus rhythm with no peaked T waves. She is haemodynamically stable and denies chest pain. What is the most appropriate next step in management?