A patient with a long-standing intrauterine contraceptive device (IUCD) develops chronic pelvic pain. The device is removed, and a biopsy of the endometrium is performed. The biopsy specimen shows a prominent infiltrate composed of lymphocytes, plasma cells, and histiocytes. Which of the following is the most likely diagnosis?

**Core Concept**
Intrauterine contraceptive devices (IUCDs) can cause an abnormal immune response in some women, leading to a condition known as **endometritis**. This inflammation of the endometrium can result in chronic pelvic pain and is often accompanied by an increase in immune cells, including lymphocytes, plasma cells, and histiocytes.

**Why the Correct Answer is Right**
The presence of a lymphocytic infiltrate in the endometrium after IUCD removal is suggestive of **endometrial inflammation**. The immune cells present in the biopsy specimen are indicative of a chronic inflammatory process. This inflammation is likely caused by the foreign body reaction to the IUCD, which can lead to an immune response and subsequent endometrial damage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **endometrial hyperplasia** is typically characterized by an increase in glandular epithelial cells, not a lymphocytic infiltrate.

**Option B:** This option is incorrect because **endometrial carcinoma** is a malignant process that would not be directly related to the removal of an IUCD or the presence of immune cells in the endometrium.

**Option C:** This option is incorrect because **endometriosis** is a condition characterized by the growth of endometrial tissue outside the uterus, which is not directly related to the presence of immune cells in the endometrium.

**Option D:** This option is incorrect because **salpingitis isthmica nodosa** is a condition characterized by the presence of nodular thickening of the fallopian tube, which is not directly related to the presence of immune cells in the endometrium.

**Clinical Pearl / High-Yield Fact**
Remember that the presence of a lymphocytic infiltrate in the endometrium after IUCD removal is a classic finding in **endometritis**, and this diagnosis should be considered in any patient with chronic pelvic pain and a history of IUCD use.

**Correct Answer:** C.

✓ Correct Answer: C. Chronic endometritis