Which is an example of an irregular bone?

• A. Radius
• B. Sacrum
• C. Scapula
• D. Patella

Answer: (B)

Irregular bones have complex shapes that don’t fit the usual categories of long, short, flat, or sesamoid. The sacrum is irregular because it’s a triangular bone at the base of the spine formed by the fusion of five sacral vertebrae, with a varied surface and foramina for nerves and vessels. Its shape supports the pelvis and helps transfer weight from the spine to the pelvis, while its irregular surfaces articulate with the ilium and coccyx. This distinguishes it from a radius (a long bone with a cylindrical shaft), a scapula (a flat bone with a broad, flat surface), and a patella (a small sesamoid bone embedded in a tendon).