A 48-hour-old preterm neonate (35 weeks gestation) born to an Rh-negative mother (unsensitized) presents with jaundice. Serum bilirubin is 12 mg/dL. Direct bilirubin is 0.8 mg/dL. Hemoglobin is 16 g/dL, reticulocyte count is 8%, and peripheral smear shows spherocytes and polychromasia. What is the most appropriate next step in management?