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
Watery nature of the cervical mucus at the time of ovulation may be caused by: 
Select and explain. 
A. Oestrogen, B. Progesterone, C. Infection, D. All of the above
A: Oestrogen
Answer: A: Oestrogen 
Explanation: Estrogen makes the mucus much thinner and more alkaline, changes that promote the survival and transpo of sperm. Progesterone makes it thick, tenacious, and cellular. The mucus is thinnest at the time of ovulation, and its elasticity, or spinnbarkeit, increases so that by midcycle a drop can be stretched into a long, thin thread that may be 8-12 cm or more in length. Ref: Alford C., Nurudeen S. (2013). Chapter 4. Physiology of Reproduction in Women. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds),CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.