Which structure is NOT part of the rib cage?

The lumbar vertebrae are not part of the rib cage. The rib cage is primarily composed of the sternum, costal cartilage, and the thoracic vertebrae. The sternum serves as the central bone that anchors the ribs at the front, while the costal cartilage connects the ribs to the sternum, providing flexibility and support. The thoracic vertebrae are located in the upper back and provide attachment points for the ribs, forming part of the rib cage structure. In contrast, the lumbar vertebrae are located in the lower back and are not involved in the rib cage's construction or function. This distinction makes the lumbar vertebrae the correct choice as they do not contribute to the rib cage's structure.