The rings of matrix within an osteon are called:

A. collagen
Collagen provides tensile strength and structural support in connective tissues but is not water-resistant or as hard as keratin.

B. elastic
Elastic fibers allow tissues to stretch and recoil. They are not hard or water-resistant.

C. hyaluronic acid fibrin
Hyaluronic acid is a glycosaminoglycan important for hydration in connective tissue; fibrin is a clotting protein. Neither is a hard, water-resistant structural protein.

D. keratin
Keratin is a fibrous protein found in skin, hair, and nails. It provides a tough, water-resistant barrier on the outer skin (especially in the stratum corneum).

A. in the medullary cavity of a long bone
In adults, the medullary cavity contains yellow marrow, not red marrow.

B. in the epiphysis of a flat bone
Flat bones (e.g., skull) don’t have epiphyses. This answer is anatomically inaccurate.

C. in the medullary cavity of a flat bone
Flat bones don’t have a true medullary cavity. Red marrow is found in their spongy bone, not in a central cavity.

D. in the epiphysis of a long bone
Red marrow in adults is found in the spongy bone of the epiphyses of long bones and in flat bones like the sternum and pelvis.

E. in the epiphyseal plate of a long bone
The epiphyseal plate is made of cartilage, not marrow. It is involved in growth, not hematopoiesis.