**Core Concept**
The most common side effect of Intrauterine Contraceptive Device (IUCD) that necessitates its removal is pain, which can manifest as dysmenorrhea or pelvic pain.

**Why the Correct Answer is Right**
Pain is the leading cause of IUCD removal due to its association with increased uterine contractions, which can lead to pelvic pain and discomfort. The presence of a foreign body in the uterus can stimulate the uterine muscle, causing it to contract more forcefully, thereby exacerbating the pain. This is often seen with the use of copper IUCDs, which release ions that stimulate the uterine muscle. The mechanism involves the release of prostaglandins and other inflammatory mediators that sensitize the uterine muscle to pain.

**Why Each Wrong Option is Incorrect**
**Option A:** While bleeding is a common side effect of IUCDs, particularly in the initial months after insertion, it is not the most common reason for removal. Most cases of bleeding are manageable with medical intervention and do not necessitate removal of the IUCD.
**Option C:** Pelvic infection is a potential complication of IUCD use, but it is relatively rare and not the most common reason for removal. Infections can be treated with antibiotics, and removal of the IUCD is usually reserved for cases where the infection is severe or recurrent.
**Option D:** Ectopic pregnancy is a known risk associated with IUCD use, but it is extremely rare and not the most common reason for removal. Ectopic pregnancies are typically diagnosed and managed with medical or surgical intervention, and removal of the IUCD is not usually necessary.

**Clinical Pearl / High-Yield Fact**
It is essential to counsel patients about the potential risks and benefits of IUCD use, including the risk of pain and the need for removal in some cases. Regular follow-up and monitoring can help identify patients who may require removal due to pain or other complications.

**✓ Correct Answer: B. Pain**