**Question:** Posterior iliac horns are seen in -

A. Lumbar spine
B. Sacrum
C. Sacroiliac joint
D. Sacrum and coccyx

**Core Concept:** Posterior iliac horns are bony projections located on the posterior aspect of the iliac bones in the pelvis. They are part of the greater sciatic notch and serve as attachment sites for muscles and ligaments.

**Why the Correct Answer is Right:** The correct answer, option D, involves the combination of two key structures in the pelvic region:

1. **Sacrum:** A fused vertebral segment forming the posterior part of the pelvis. It consists of five fused vertebrae (S1, S2, S3, S4, S5) and contributes to the formation of the sacroiliac joint and coccygeal region.
2. **Coccyx (tailbone):** A small, bony structure at the base of the spine, composed of four or five fused vertebrae (C0-C4 or C0-C4 and C0-C5).

**Why Each Wrong Option is Incorrect:**

1. **Option A (Lumbar spine):** The lumbar spine is located in the lower back, not in the pelvis. Posterior iliac horns are part of the pelvic region.
2. **Option B (Sacrum only):** The sacrum alone is not sufficient to form the posterior aspect of the pelvic region and the attachment sites of muscles and ligaments.
3. **Option C (Coccyx only):** The coccyx is not responsible for the formation of the sacroiliac joint and the posterior aspect of the pelvic region.

**Clinical Pearl:** Understanding the location and function of posterior iliac horns is essential for understanding pelvic anatomy, orthopedic injuries, and their treatment. A thorough knowledge of these structures aids in diagnosing and treating conditions such as sacroiliac joint dysfunction, coccydynia, and pelvic floor dysfunction. Additionally, knowledge of these structures helps in identifying the source of referred pain from these regions.