**Question:** Rider's bones are ossification of

A. Cervical vertebrae
B. Thoracic vertebrae
C. Lumbar vertebrae
D. Sacrum

**Core Concept:** Rider's bones are a set of five small vertebrae that ossify (develop bone) during the fetal period and contribute to the formation of the sacrum and coccyx.

**Why the Correct Answer is Right:** The correct answer, D (Sacrum), is derived from the fused posterior elements of the lumbar vertebrae (L4-L5) and the fusion of the coccygeal vertebrae (C4-C5). These fused bones help provide stability and support to the pelvis and form part of the bony structure of the sacrum.

**Why Each Wrong Option is Incorrect:**

A. Cervical vertebrae (Cervical vertebrae ossify into cervical ribs, which are not related to Rider's bones)
B. Thoracic vertebrae (Thoracic vertebrae ossify into the ribs and sternum, not Rider's bones)
C. Lumbar vertebrae (As explained above, the correct answer; lumbar vertebrae ossify into the sacrum and coccyx, not Rider's bones)

**Clinical Pearl:** Rider's bones are an essential component of the human vertebral column, contributing to the formation of the sacrum and coccyx, which provide structural support and stability to the pelvic region.

**Correct Answer:** D. Sacrum