This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood: RBCs, Anemia, and Polycythemia, Subject name: Physiology
Up to 6 weeks of gestation erythropoiesis occurs in? 
Select and explain. 
A. Yolk sac, B. Bone marrow, C. Spleen, D. Liver
A: Yolk sac
Answer: A: Yolk sac 
Explanation: ANSWER: (A) Yolk sacREF: Guyton, 6th edition, page 420, Wintrobe s clinical hematology 12th ed page 90, William's hematology 8th ed chapter 6See details of fetal erythropoiesis in Physiology December 2011