A large, triangular bone at the base of the spine.

• A. Sacrum
• B. Coccyx
• C. Lumbar Vertebra
• D. Pelvis

Answer: (A)

The sacrum is the large triangular bone at the base of the spine. It sits between the two hip bones and forms the posterior part of the pelvis. This bone results from the fusion of five sacral vertebrae and acts as a keystone that distributes weight from the spine into the pelvis and legs. It connects upward with the last lumbar vertebra and downward with the coccyx, and its sides articulate with the ilium bones at the sacroiliac joints. The coccyx, in contrast, is the small tailbone at the very bottom; the lumbar vertebrae are the individual bones of the lower back above the sacrum; and the pelvis is the ring of hip bones, not a single bone.