You are answering a USMLE-style medical question. Question: A 2-day-old male newborn is brought to the physician because of yellowing of the skin and sclerae for 16 hours. He had previously been well. He was born at 38 weeks' gestation via uncomplicated vaginal delivery and weighed 3.1 kg (6 lb 13 oz). The mother has no medical insurance and did not receive prenatal care. The newborn's 4-year-old brother has sickle cell disease. Examination shows jaundice. The abdomen is mildly distended. The liver is palpated 1 cm below the right costal margin and the spleen tip is palpated just below the left costal margin. Laboratory studies show: Hemoglobin 11 g/dL Reticulocytes 9% Leukocytes 9,100/mm3 Platelets 244,000/mm3 Maternal blood group 0, Rh-negative Anti-Rh antibody titer positive Fetal blood group B, Rh-negative Serum Bilirubin, total 11.3 mg/dL Direct 0.3 mg/dL Which of the following is the most likely cause of this patient's condition?" Options: A) RBC sickling B) Anti-D antibodies C) Biliary duct malformation D) Anti-B antibodies Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.