Increased LH secretion just before ovulation is due to
**Core Concept**
The regulation of the menstrual cycle involves a complex interplay between various hormones, including luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and progesterone. The surge in LH secretion just before ovulation is a critical event that triggers ovulation and subsequent fertilization.
**Why the Correct Answer is Right**
Estrogen plays a pivotal role in the positive feedback mechanism that leads to the LH surge. As the dominant follicle grows, it secretes increasing amounts of estrogen, which in turn stimulates the hypothalamus to release gonadotropin-releasing hormone (GnRH). GnRH then stimulates the pituitary gland to release LH in a surge, resulting in ovulation. This positive feedback loop is essential for the timely release of the ovum from the follicle. The increase in estrogen levels creates a self-reinforcing cycle, amplifying the signal to the pituitary gland and ultimately leading to the LH surge.
**Why Each Wrong Option is Incorrect**
**Option A:** Progesterone does not play a role in the positive feedback mechanism leading to the LH surge. Instead, it has a negative feedback effect on GnRH and LH release, helping to regulate the menstrual cycle.
**Option C:** FSH is essential for follicular growth and development, but it does not directly stimulate the LH surge. FSH and LH are separate hormones with distinct functions, and the LH surge is primarily driven by the positive feedback of estrogen.
**Option D:** Relaxin is a hormone produced by the corpus luteum and the placenta during pregnancy, but it is not involved in the positive feedback mechanism leading to the LH surge.
**Clinical Pearl / High-Yield Fact**
The timing of the LH surge is critical for ovulation, and any disruption in this process can lead to infertility or other reproductive issues. Understanding the role of estrogen in the positive feedback mechanism is essential for managing conditions such as polycystic ovary syndrome (PCOS) or infertility.
**✓ Correct Answer: B. Positive feed-back by estrogen**