**Core Concept**
Endometriosis is a chronic inflammatory condition characterized by the presence of endometrial-like tissue outside the uterus, leading to pelvic pain and infertility. The ectopic endometrial tissue responds to hormonal changes, causing inflammation and scarring.

**Why the Correct Answer is Right**
The most common symptom of endometriosis is pelvic pain, specifically dysmenorrhea, which is severe menstrual cramping. This pain is thought to be caused by the release of prostaglandins from the ectopic endometrial tissue, leading to uterine contractions and inflammation. The pain may be accompanied by dyspareunia (painful intercourse) and dyschezia (painful bowel movements).

**Why Each Wrong Option is Incorrect**
* **Option A:** **Bleeding between periods** is a possible symptom, but not the most common one. Endometriosis-related bleeding is often irregular and may be accompanied by heavy menstrual flow or intermenstrual bleeding.
* **Option B:** **Infertility** is a significant complication of endometriosis, but it is not the most common symptom. While endometriosis can lead to infertility, it is often asymptomatic or presents with pelvic pain.
* **Option C:** **Abdominal mass** is not a typical symptom of endometriosis. While large endometriomas (cysts) may form, they are not usually palpable as an abdominal mass.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that endometriosis often presents with a combination of symptoms, including pelvic pain, dysmenorrhea, and infertility. A high index of suspicion is necessary for diagnosis, especially in women with a history of pelvic pain and infertility.

**Correct Answer: C. Abdominal mass**