**Endometriosis and Infertility**

**Core Concept**
Endometriosis is a chronic gynecological disorder characterized by the growth of endometrial tissue outside the uterus, leading to inflammation, scarring, and adhesions. This condition can significantly affect reproductive function, resulting in infertility and other symptoms.

**Why the Correct Answer is Right**
The prevalence of endometriosis in women with unexplained infertility is estimated to be around 30-50%. The exact mechanism behind endometriosis-induced infertility is not fully understood, but it is believed to be related to the inflammatory environment created by endometrial implants, which can disrupt ovulation, fertilization, and implantation. The presence of endometriosis can also lead to adhesions and scarring in the pelvic cavity, further impeding fertility.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as there is no established prevalence of endometriosis in women with unexplained infertility below 20%.
**Option B:** This option is incorrect as the prevalence of endometriosis in women with unexplained infertility is not less than 20%.
**Option C:** This option is incorrect as the prevalence of endometriosis in women with unexplained infertility is not less than 50%.

**Clinical Pearl / High-Yield Fact**
A significant proportion of women with unexplained infertility have endometriosis, highlighting the importance of considering endometriosis in the evaluation and management of infertility.

**Correct Answer: C. 50%**