Irregular bones are characterized by which internal composition?

• A. Dense compact bone
• B. Primarily spongy bone covered with a thin layer of compact bone
• C. Porous marrow
• D. Mostly cartilage

Answer: (B)

Irregular bones combine a lattice-like interior with a thin outer shell to balance strength and lightness. The inside is primarily spongy (cancellous) bone, a porous network of trabeculae that creates spaces for red marrow and allows the bone to absorb stresses from multiple directions. This spongy core is surrounded by a thin layer of dense compact bone that provides the necessary rigidity and surface strength. That combination fits irregular bones like vertebrae and certain facial bones, which experience forces from many directions and still need to be relatively lightweight. Dense compact bone throughout would be too heavy and less able to distribute multi-directional loads; marrow alone doesn’t describe the structural pattern; and bone is not composed mostly of cartilage.