A 32-year-old woman presents with severe dysmenorrhea and chronic pelvic pain. Imaging reveals multiple cystic lesions in the ovary. What is the most common site of endometriosis?

Explanation

## Most Common Site of Endometriosis **Key Point:** The ovary is the most common site of endometriosis, accounting for approximately 35–40% of all endometriotic lesions. These typically present as chocolate cysts (endometriomas) on imaging. ### Distribution of Endometriosis Sites | Site | Frequency (%) | Clinical Presentation | |------|---------------|----------------------| | Ovary | 35–40 | Chocolate cysts, dysmenorrhea | | Uterosacral ligament | 20–25 | Nodular lesions, pelvic pain | | Pouch of Douglas | 15–20 | Posterior cul-de-sac involvement | | Fallopian tube | 5–10 | Tubal obstruction, infertility | | Peritoneum | 10–15 | Superficial lesions | **High-Yield:** The ovary is involved in >50% of women with endometriosis because of its rich vascularization and proximity to the peritoneal cavity. Endometriomas are often bilateral (20–30% of cases). **Clinical Pearl:** Chocolate cysts derive their name from the hemosiderin-stained appearance of old blood products within the cyst. They are pathognomonic for ovarian endometriosis. **Mnemonic:** **OPUF** — Ovary (most common), Peritoneum, Uterosacral ligament, Fallopian tube — in order of frequency. ### Why Ovary? 1. **Anatomical proximity** — The ovary is in direct contact with the peritoneal cavity. 2. **Retrograde menstruation** — Menstrual debris preferentially deposits on ovarian surface. 3. **Angiogenesis** — Ovarian tissue supports neovascularization and ectopic endometrial growth. [cite:TW Bourne Benign Gynecology, DeCherney Gynecology 14e Ch 9]