Spongy bone contains:

Choice A reason: Osseous matrix refers to the mineralized extracellular matrix of bone tissue but is not the structural unit.

Choice B reason: Trabeculae are the structural units of spongy bone, not compact bone.

Choice C reason: Sarcomeres are the contractile units of muscle fibers, not bone.

Choice D reason: Osteon is the correct answer. Also known as the Haversian system, it is the cylindrical structure in compact bone that contains a central canal surrounded by concentric lamellae, housing blood vessels and nerves.

Choice E reason: Lamellar bone refers to mature bone with organized collagen fibers. While osteons are made of lamellae, the osteon is the functional unit.

Choice A reason: Flat bones are typically thin and broad, such as the sternum or skull bones. They do not match the described dimensions of Emily’s bone.

Choice B reason: Irregular bones have complex shapes that do not fit into other categories, such as vertebrae. The bone described is not irregular.

Choice C reason: Short bones are roughly cube-shaped and found in areas like the wrist and ankle. A bone that is 3 inches long does not fit this category.

Choice D reason: Long bones are characterized by being longer than they are wide, typically found in the limbs. A bone that is 3 inches long and 1 inch wide fits this classification.

Choice E reason: Sesamoid bones are small, round bones embedded in tendons, like the patella. The described bone does not fit this category