**Core Concept**
An intra-uterine device (IUD) is a form of long-term reversible contraception that involves inserting a small device into the uterus. The absolute contraindication to IUD placement is a critical consideration in clinical practice, particularly in patients with certain medical conditions. The underlying principle being tested is the identification of a condition that precludes the safe insertion and use of an IUD.

**Why the Correct Answer is Right**
The correct answer is **C. Active pelvic inflammatory disease (PID)**. This is because active PID increases the risk of IUD-related complications, such as increased risk of pelvic abscesses and infertility. The IUD can also exacerbate the infection, leading to severe consequences. The mechanism behind this contraindication is the increased risk of bacterial colonization and subsequent infection at the IUD site.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while a history of PID may be a relative contraindication, it does not preclude the use of an IUD. Patients with a history of PID may still be candidates for IUD placement after a certain period of time has elapsed.
* **Option B:** This option is incorrect because a history of ectopic pregnancy is not an absolute contraindication to IUD placement. However, it is essential to consider the patient's reproductive history and the risk of future ectopic pregnancies.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the absolute contraindications to IUD placement are relatively rare, and most women with medical conditions can still be candidates for IUD placement. A thorough medical history and physical examination are crucial in determining the suitability of an IUD for each patient.

**Correct Answer:** C. Active pelvic inflammatory disease (PID).