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
[{'content': 'What is the mechanism of action of Fulvestrant? \nSelect and explain. \nA. Selective estrogen agonist, B. Selective esrogen receptor modulator, C. Selective esrogen receptor upregulator, D. Selective esrogen receptor downregulator', 'role': 'user'}, {'content': "Answer: D \nExplanation: Fulvestrant: is an estrogen receptor antagonist that downregulates the estrogen receptor and has no agonist effects. Fulvestrant competitively inhibits binding of oestradiol to the estrogen receptor (ER), with a binding affinity that is 89% that of oestradiol. Reduces the number of estrogen receptor. Good to know: It is approved for the treatment of metastatic breast cancer in postmenopausal women who have failed antiestrogen therapy. Given once in a month as a intramusclar injection. Ref: Goodman & Gillman's Pharmacological Basis of Therapeutics' 12th Edition, Page 1759, 1760.", 'role': 'assistant'}]