This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Female Reproductive System, Subject name: Physiology
As menstruation ends, estrogen levels in the blood rise rapidly. What is the source of the estrogen? 
Select and explain. 
A. Corpus luteum, B. Developing follicles, C. Endometrium, D. Stromal cells of the ovaries
B: Developing follicles
Answer: B: Developing follicles 
Explanation: In the nonpregnant female, the only significant source of estrogen is ovarian follicles or corpus luteum. Menstruation begins when the corpus luteum degenerates. Menstruation ends when developing follicles secrete estrogen sufficiently to raise circulating concentration to a level that stimulates regrowth of the endometrium