**Question:** True about endometriosis is/are:

A. It is a benign tumor.
B. It primarily affects women of reproductive age.
C. It is characterized by the presence of endometrial tissue outside the uterus.
D. It is a rare condition.

**Core Concept:**
Endometriosis is a chronic inflammatory disorder characterized by the presence of endometrial tissue (the tissue lining the uterus) outside the uterus, most commonly in the pelvic cavity. This ectopic tissue exhibits menstrual-like bleeding, leading to inflammation and pain, and can potentially cause infertility.

**Why the Correct Answer is Right:**
C is the correct answer because endometriosis is characterized by the presence of endometrial tissue outside the uterus, specifically in the pelvic cavity. This ectopic tissue undergoes menstrual-like bleeding, causing inflammation and pain. While endometriosis primarily affects women of reproductive age (option B), it is not necessarily a benign tumor (A) as it can progress and cause significant morbidity. Endometriosis is relatively common but not extremely rare (D).

**Why Each Wrong Option is Incorrect:**
A) Endometriosis is not a benign tumor because it can progress and cause significant morbidity. Benign tumors are typically non-cancerous and do not invade surrounding tissues.
B) Although endometriosis primarily affects women of reproductive age, it can occur at any age, making D the correct answer.
D) Endometriosis is relatively common but not extremely rare, making D the correct answer.

**Clinical Pearl:**
Endometriosis can present with a wide range of symptoms, including dysmenorrhea (severe menstrual cramps), dyspareunia (sexual pain), dyschezia (bowel-related symptoms), and pelvic pain. Early diagnosis and treatment are essential to prevent complications such as adhesions, fertility issues, and pelvic pain.