Which structure contributes to the upper appendicular skeleton?

The pectoral and shoulder girdle is indeed the correct choice as it forms an essential part of the upper appendicular skeleton. This structure is composed of the clavicles (collarbones) and the scapulae (shoulder blades). It plays a pivotal role in connecting the upper limbs to the trunk, allowing for a wide range of motion and mobility in the arms. The pectoral girdle also provides attachment points for numerous muscles that facilitate the movement of the shoulders and arms.

The other options include structures that are associated with different parts of the skeleton. The pelvic girdle, for example, supports the lower appendicular skeleton and is involved in weight-bearing and locomotion of the lower limbs. The lower extremities refer to the bones of the legs and feet, while the vertebral column is part of the axial skeleton and primarily protects the spinal cord, providing structural support and flexibility to the body. Thus, the upper appendicular skeleton is specifically defined by the inclusion of the pectoral and shoulder girdle.