This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
[{'content': 'Which organ is the primary site of hematopoiesis in the fetus before mid-pregnancy – \nSelect and explain. \nA. Bone, B. Liver, C. Spleen, D. Lung', 'role': 'user'}, {'content': 'Answer: B \nExplanation: In fetus, hematopoiesis occurs in three anatomic stages.\nMesoblastic stage\xa0\n\nThis stage of hematopoiesis begins between 10th and 14th days of gestation\nIt occurs in the extraembryonic structures principally in the yolk sac.\nThis stage of hematopoiesis essentially ceases by 10-12 week of gestation.\n\nHepatitis stage\n\nBy 6-8 weeks of gestation, the liver replaces the yolk sac as the primary site of gestation.\nHematopoiesis occurs in the liver throughout the remainder of the gestation and the liver remains the predominant hematopoietic organ, till 20-24 weeks of the gestation.\nAfter this, the hematopoiesis begins to decrease in liver and in full-term infant hepatic hematopoiesis is seen only in widely scattered small foci which soon become inactive after birth.\n\nMyeloid stage\n\nHematopoiesis in the marrow starts by the 4th month of gestation.\nBy birth, marrow throughout the skeleton is hematopoietically active and virtually the sole source of blood cells.\n\nHematopoiesis after birth\nUpto puberty -\xa0Hematopoiesis occurs throughout the skeleton\nAfter puberty\xa0(By the age of 18) -\xa0Only the vertebrae, ribs, sternum, skull, pelvis and proximal epiphyseal regions of the\xa0humerus and femur retain red marrow, the remaining marrow becomes yellow, fatty an inactive. Thus, in adults only about half of the marrow space is active-in hematopoiesis.', 'role': 'assistant'}]