A 25-year-old woman has a 28-day menstrual cycle. On which day does the LH surge occur, and what is the mechanism by which this LH surge triggers ovulation?

• A Day 14; LH surge triggers oocyte meiosis resumption (completion of meiosis I), prostaglandin synthesis, and proteolytic enzyme activation causing follicular wall rupture ✓
• B Day 28; LH surge increases progesterone, which ruptures the follicle
• C Day 7; LH surge triggers follicular atresia of non-dominant follicles
• D Day 21; the LH surge follows the progesterone rise in the luteal phase

Explanation

In a 28-day cycle, the LH surge occurs on approximately day 13–14, triggered by the positive feedback effect of rising estrogen (from the dominant follicle reaching a threshold of ~200 pg/mL for ~48 hours) on the hypothalamo-pituitary axis. The LH surge (1) resumes meiosis I in the primary oocyte (which was arrested in prophase I since fetal life); (2) stimulates synthesis of prostaglandins and proteolytic enzymes in the follicle wall; (3) triggers angiogenesis and histamine release — all causing the follicular wall to thin and rupture approximately 36–40 hours after the LH surge peak.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.