Which of the following is NOT a main component of the skeletal system?

The skeletal system is primarily made up of bones that provide structure, support, and protection for the body, as well as facilitating movement in conjunction with muscles. The pelvis, shoulder girdle, and femur are all integral components of the skeletal system. The pelvis consists of several bones and forms the base of the spine, the shoulder girdle connects the arms to the trunk, and the femur is the thigh bone, which is the longest bone in the body.

In contrast, the heart is a vital organ of the circulatory system, responsible for pumping blood throughout the body, and is not classified as a part of the skeletal system. Hence, it does not contribute to the structural framework or the primary functions associated with the skeletal system, such as support or movement. This distinction makes the heart the correct answer as the element that is not a main component of the skeletal system.