Withdrawal of Progesterone is the immediate cause of menstruation.

Explanation:
Menstruation is a part of the menstrual cycle and occurs in response to hormonal changes in the female reproductive system. The menstrual cycle is controlled by a delicate balance of hormones that regulate the growth and shedding of the uterine lining. One of the key hormones involved in this process is progesterone.

Progesterone is primarily produced by the ovaries during the second half of the menstrual cycle, specifically during the luteal phase. Its main function is to prepare the uterus for pregnancy by thickening the uterine lining, known as the endometrium. Progesterone also inhibits the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which prevents the development of additional follicles and ovulation.

During the menstrual cycle, if pregnancy does not occur, the corpus luteum, which is responsible for producing progesterone, begins to break down. As a result, the progesterone levels in the body decrease. This drop in progesterone triggers a series of events that lead to menstruation.

1. Withdrawal of Progesterone: The decrease in progesterone levels causes the blood vessels that supply the endometrium to constrict, reducing the blood flow to the tissue. This leads to the shedding of the uterine lining.

2. Menstruation: The shedding of the endometrium results in the release of blood and tissue through the vagina, which is known as menstruation.

3. FSH and LH Surge: The decrease in progesterone levels also signals the hypothalamus to release gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to produce and release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

4. Follicular Phase: The rise in FSH levels initiates the development of several follicles in the ovaries. These follicles produce estrogen, which stimulates the regrowth of the endometrium.

5. Ovulation: As the follicles mature, they release increasing amounts of estrogen. When estrogen levels peak, it triggers a surge in LH, which causes the release of an egg from the ovary, known as ovulation.

In summary, the withdrawal of progesterone, along with the subsequent rise in FSH and LH levels, is the immediate cause of menstruation. The decrease in progesterone leads to the shedding of the uterine lining, while the increase in FSH and LH initiates the development of new follicles and prepares the body for the next menstrual cycle.