Which of the following is NOT part of the axial skeleton?

The femur is classified as a long bone in the appendicular skeleton, which includes the bones of the limbs and the girdles that attach them to the axial skeleton. In contrast, the axial skeleton comprises the skull, spine (vertebral column), and rib cage, which are primarily responsible for protecting the central nervous system and the thoracic organs. The skull protects the brain, the spine supports the body and encases the spinal cord, and the sternum, along with the ribs, protects vital organs within the thorax while also providing structural support. Thus, the femur does not belong to the axial skeleton and is instead a key component of the appendicular skeleton, which is why it is the correct answer.