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
Which of the following is the main form of oestrogen secreted from the ovary? 
Select and explain. 
A. Estradiol, B. Estrone, C. Estriol, D. All of the above
A: Estradiol
Answer: A: Estradiol 
Explanation: The major estrogens produced by women are estradiol (estradiol-17, E2), estrone (E1), and estriol (E3). Estradiol is the major secretory product of the ovary. Although some estrone is produced in the ovary, most estrone and estriol are formed in the liver from estradiol or in peripheral tissues from androstenedione and other androgens. During the first pa of the menstrual cycle estrogens are produced in the ovarian follicle by the theca and granulosa cells. After ovulation, the estrogens as well as progesterone are synthesized by the luteinized granulosa and theca cells of the corpus luteum, and the pathways of biosynthesis are slightly different. Ref: Chrousos G.P. (2012). Chapter 40. The Gonadal Hormones & Inhibitors. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds), Basic & Clinical Pharmacology, 12e.