**Core Concept**
The most common side effect of Intrauterine Device (IUD) insertion is a significant inflammatory response, which can lead to pelvic inflammatory disease (PID). This is typically caused by the migration of bacteria from the vagina into the upper genital tract.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of IUD insertion and the subsequent inflammatory response. When an IUD is inserted, it can cause micro-trauma to the uterine lining, leading to an increase in prostaglandin synthesis. This results in increased uterine contractions, inflammation, and potentially PID. The most common organisms responsible for PID in IUD users are Neisseria gonorrhoeae and Chlamydia trachomatis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while bleeding and spotting are common side effects of IUD insertion, they do not typically require removal of the device.
* **Option B:** This option is incorrect because while infection is a potential side effect of IUD insertion, it is not the most common reason for device removal.
* **Option C:** This option is incorrect because while perforation is a rare but serious complication of IUD insertion, it is not the most common reason for device removal.

**Clinical Pearl / High-Yield Fact**
It is essential to screen for sexually transmitted infections (STIs) before inserting an IUD to reduce the risk of PID. A thorough medical and surgical history should also be obtained to identify any potential contraindications to IUD insertion.

**Correct Answer: B. Infection.