Which of the following is NOT one of the four layers of bone?

• A. Periosteum
• B. Compact bone
• C. Cancellous bone
• D. Densified tissue

Answer: (D)

Bone has distinct structural layers that form its outer and inner architecture. The outside is covered by a fibrous membrane called the periosteum; beneath that lies the dense, hard layer known as compact bone; inside this dense shell is the porous network called cancellous (spongy) bone, which contains spaces often filled with marrow; the inner lining that coats the cavities and trabeculae is the endosteum. A term like “densified tissue” isn’t a recognized bone layer and doesn’t belong with these real components. So the option that doesn’t fit is densified tissue.