In the normal female reproductive cycle, menstruation is primarily triggered by a sharp drop in the blood levels of which hormone?

Introduction / Context:
The menstrual cycle is regulated by a complex interaction of hormones produced by the hypothalamus, pituitary gland, and ovaries. Different phases of the cycle are associated with changing levels of estrogen and progesterone. Menstruation, or the shedding of the uterine lining, occurs when the hormonal support for that lining is withdrawn. This question asks which specific hormone drop is most directly associated with the onset of menstruation.

Given Data / Assumptions:

• The focus is on the hormonal trigger for menstruation.
• Options include progesterone, relaxin, follicle stimulating hormone, and luteinising hormone.
• We assume knowledge of the luteal phase and the role of the corpus luteum.

Concept / Approach:
After ovulation, the ruptured follicle in the ovary transforms into the corpus luteum, which secretes large amounts of progesterone and some estrogen. Progesterone prepares and maintains the endometrium (uterine lining) for possible implantation of a fertilised egg. If fertilisation and implantation do not occur, the corpus luteum degenerates. As it degenerates, progesterone levels fall sharply. This drop in progesterone, along with falling estrogen, leads to constriction of uterine blood vessels and breakdown of the functional layer of the endometrium, resulting in menstrual bleeding.

Step-by-Step Solution:
Step 1: Identify the phase just before menstruation as the late luteal phase, when the corpus luteum is regressing in a non pregnant cycle. Step 2: Recall that the corpus luteum mainly secretes progesterone, with some estrogen, to maintain the endometrial lining. Step 3: Understand that when the corpus luteum degenerates, its hormone output declines sharply, especially progesterone. Step 4: Recognise that this sharp fall in progesterone is the key trigger that leads to shedding of the uterine lining, that is menstruation, and therefore select progesterone as the correct answer.

Verification / Alternative check:
Clinical uses of synthetic progesterone support this explanation. For example, withdrawal bleeding in some hormonal treatments occurs when progesterone containing medication is stopped, causing a sudden drop in progesterone levels. This withdrawal bleeding mimics natural menstruation. Such observations confirm the central role of progesterone withdrawal in initiating menstrual bleeding.

Why Other Options Are Wrong:

• Relaxin: This hormone is more relevant in pregnancy, where it helps soften ligaments and cervix; it is not the main trigger of menstruation.
• Follicle stimulating hormone: FSH is important for follicular development in the early cycle but does not directly cause the shedding of the endometrium.
• Luteinising hormone: LH triggers ovulation and supports corpus luteum formation but the onset of menstruation is linked more directly to falling progesterone when the corpus luteum regresses.

Common Pitfalls:
Students can be confused by the many hormones involved in the cycle. A useful memory aid is to link progesterone with pregnancy support and maintenance of the uterine lining. When this hormone falls, the lining cannot be maintained and menstruation follows. It is also important not to confuse the ovulation trigger (LH surge) with the menstruation trigger (progesterone fall), since they occur at different times in the cycle.

Final Answer:
Menstruation is primarily triggered by a sharp drop in progesterone levels when the corpus luteum regresses in a non pregnant cycle.