**Core Concept**
Immediately prior to ovulation, a sharp rise in luteinizing hormone (LH) occurs, triggering the final maturation and release of the oocyte from the follicle. This event is a key part of the menstrual cycle's regulation and is tightly controlled by negative feedback loops involving sex hormones.

**Why the Correct Answer is Right**
The **LH surge** occurs in the late follicular phase, typically 24–36 hours before ovulation. It is triggered by a sustained rise in **estrogen** from the maturing follicle, which removes negative feedback on the hypothalamus and pituitary. The surge causes the granulosa cells to release **LH**, leading to the rupture of the mature follicle and ovulation. This is a well-defined, time-bound physiological event in human reproductive physiology.

**Why Each Wrong Option is Incorrect**
Option B: FSH surge – FSH levels peak earlier in the cycle and decline before ovulation; it does not surge immediately prior to ovulation.
Option C: Progestrone surge – Progestrone rises after ovulation and during the luteal phase, not before.
Option D: Estrogen surge – While estrogen rises significantly before ovulation, it is the **LH surge** that directly triggers ovulation, not the estrogen surge itself. Estrogen is the *trigger* for LH, but the *hormonal activity* immediately prior to ovulation is the **LH surge**.

**Clinical Pearl / High-Yield Fact**
The **LH surge** is the definitive marker for ovulation and is used clinically in fertility monitoring (e.g., ovulation predictor kits). A rise in LH precedes ovulation by 24–36 hours and is the key event that causes follicular rupture.

✓ Correct Answer: A. LH surge