Biological Systems MCAT Practice Exam

What triggers the start of menstruation in the uterine cycle?

1. High levels of progesterone

2. Degeneration of the corpus luteum

3. Maturation of the follicle

4. Release of the ovum

The correct answer is: Degeneration of the corpus luteum

Menstruation in the uterine cycle is triggered primarily by the degeneration of the corpus luteum. After ovulation, the corpus luteum forms from the remnants of the mature follicle and secretes hormones, particularly progesterone and estrogen. These hormones are essential for preparing the endometrium (uterine lining) for potential implantation of a fertilized egg. If pregnancy does not occur, the corpus luteum will degenerate, leading to a significant drop in progesterone and estrogen levels. The decline in these hormones causes the endometrial lining to shed, resulting in menstruation. This breakdown and shedding of the endometrial tissue mark the beginning of the menstrual cycle. The other choices relate to different stages of the menstrual and ovarian cycles rather than the initiation of menstruation. High levels of progesterone are involved in maintaining the uterine lining during the luteal phase, and maturation of the follicle and the release of the ovum are associated with ovulation, which occurs prior to the onset of menstruation.