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: Gynaecology & Obstetrics
All are fetal membranes EXCEPT : 
Select and explain. 
A. Chorion, B. Amnion, C. Decidua capsularis, D. Yolk sac
D: Yolk sac
Answer: D: Yolk sac 
Explanation: Yolk sac