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: Unknown
The yolk sac is derived from - 
Select and explain. 
A. Hypoblasts, B. Epiblasts, C. Syncytiotrophoblast, D. None
A: Hypoblasts
Answer: A: Hypoblasts 
Explanation: Ans. is 'a' i.e., Hypoblasts o The yolk sac initially appears as a transient primary yolk sac, develops from hypoblast cells. By the end of 2nd week, the lower half of primary yolk sac pinches off to form definitive yolk sac, while the second wave of hypoblast cells from the inner lining of the definitive yolk sac.