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: Pharmacology
All of the following are natural estrogens except ? 
Select and explain. 
A. Estradial, B. Ethinylestradiol, C. Estriol, D. Estrone
B: Ethinylestradiol
Answer: B: Ethinylestradiol 
Explanation: Ans. is 'b' i.e., Ethinylestradiol o Estradial, estrone and estrial, all three are found in blood, but estradiol is the most potent estrogen.