A 6-year-old boy from Delhi presents with pallor, hepatosplenomegaly, and recurrent infections. His hemoglobin is 8.5 g/dL with microcytic indices. Serum ferritin is markedly elevated at 450 ng/mL, and transferrin saturation is 65%. Peripheral blood smear shows target cells. Which investigation is most appropriate to determine the underlying cause of secondary iron overload?