What is the difference between cortical bone and spongy bone?

• A. Spongy bone forms the dense outer layer.
• B. Cortical bone is the dense outer layer, while spongy bone makes up about 20% of the skeletal mass.
• C. Both are found only in the skull.
• D. Spongy bone makes up most of the skeletal mass.

Answer: (B)

Understanding the difference between cortical and spongy bone comes down to structure, location, and mass contribution. Cortical bone is the dense outer shell that gives bones their stiffness and ability to withstand weight-bearing forces; it’s tightly packed and organized into compact layers. Spongy bone, by contrast, is a porous, lattice-like network inside the bone that makes the structure lighter and more metabolically active, while also housing marrow.

In adults, cortical bone accounts for about 80% of skeletal mass, whereas spongy bone accounts for roughly 20%. That’s why the description that cortical bone is the dense outer layer and that spongy bone makes up about 20% of skeletal mass is correct.

The other statements don’t fit because spongy bone does not form the dense outer layer, bones are not found only in the skull, and spongy bone does not constitute the majority of skeletal mass.