The sites of active red marrow differ for adults and young children. Which one site is active in children but not in adults?

a. Proximal end of the long bones
b. Skull
c. Distal ends of the long bones
d. Pelvis

ANS: C
Red marrow is found in all the bones in children, including the distal ends of the long bones. It also initially has no fat. As the child grows and the bones lengthen and enlarge, all the available marrow space is no longer needed for blood cell production. The marrow space no longer needed becomes inactive and fatty ("yellow marrow"). It can be converted to active marrow if the demand for blood cells becomes excessive. Red marrow in an adult is composed of both blood-producing cells and fat in a roughly 1:1 ratio.