Which statement correctly differentiates compact bone from spongy bone?

• A. Compact bone is dense and forms the outer layer with concentric lamellae; spongy bone (trabecular bone) is porous inside the ends of long bones and in the interior of other bones, consisting of trabeculae.
• B. Compact bone is porous inside; spongy bone is dense outer layer.
• C. Both have the same density; only location differs.
• D. Spongy bone forms outer cortex; compact bone forms interior.

Answer: (A)

The key distinction is in structure and density. Compact bone is dense and forms the outer shell of most bones, organized into tightly packed units with concentric lamellae surrounding a central (Haversian) canal. This compact arrangement provides great strength for bearing loads and withstanding pressure along the bone’s axis.

Spongy bone, on the other hand, is porous and lies inside the bone, such as the ends of long bones (epiphyses) and the interior of other bones. It’s made up of a lattice of trabeculae that create many small spaces, which lightens the bone and provides a scaffold for bone marrow, while still offering structural support where needed. The trabecular network is oriented along lines of stress, helping distribute forces.

So the statement that compact bone is dense and forms the outer layer with concentric lamellae, while spongy bone is porous inside and consists of trabeculae, accurately captures the main differences. The other options describe incorrect relationships—porosity attributed to compact bone, same density, or spongy bone forming the outer cortex—which do not fit the actual anatomy.