A 48-year-old woman with Stage 3b CKD presents with fatigue and dyspnea on exertion. Hemoglobin is 9.2 g/dL. Serum creatinine is 2.1 mg/dL, iron studies show serum iron 45 µg/dL, ferritin 180 ng/mL, and TSAT 18%. Which investigation is most appropriate to confirm the primary cause of anemia in this patient?