USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

The surge in luteinizing hormone (LH) is the critical event that triggers ovulation. During the menstrual cycle, estrogen levels increase due to the maturation of ovarian follicles. When estrogen reaches a certain threshold, it leads to a positive feedback mechanism that stimulates a sudden burst of LH from the anterior pituitary gland. This LH surge causes the mature follicle to rupture and release the ovum, marking the ovulation phase of the cycle.

This process is essential in the female reproductive system, as it allows for the possibility of fertilization and subsequent pregnancy. In contrast, the other options, while related to the overall hormonal regulation of the menstrual cycle, do not directly initiate the ovulation process. Estrogen, for instance, plays an important role in follicular development but does not itself trigger ovulation. Progesterone primarily helps to prepare the endometrium for potential implantation of a fertilized egg after ovulation has occurred. GnRH (gonadotropin-releasing hormone) is involved in stimulating the pituitary to release LH and FSH but is not the hormone that directly induces ovulation. Therefore, LH is uniquely responsible for initiating ovulation through its surge at this pivotal moment in the menstrual cycle.