This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Pharmacology
[{'content': 'All of the following are natural estrogens EXCEPT: \nSelect and explain. \nA. Estradiol, B. Ethinylestradiol, C. Estriol, D. Estrone', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ethinylestradiol is a synthetic steroidal estrogen. All others are natural estrogens. (Ref.Essentials of medical pharmacology 7th edition page no.306)', 'role': 'assistant'}]