Which area is typically not associated with the skeletal system but is involved in attachment to the torso?

The pectoral girdle is primarily considered part of the skeletal system. It consists of the clavicles and scapulae, which provide structural support for the upper limbs and facilitate their attachment to the torso. This area directly connects the upper limbs to the axial skeleton, making it a crucial element in upper limb movement and stabilization.

In contrast, the costal margin refers to the lower edge of the rib cage, the axilla is the region (armpit) that serves as the junction between the arm and the torso, and the umbilicus is the belly button, which is a remnant from fetal development where the umbilical cord was attached. Although the costal margin and axilla are anatomically close to the skeletal system, they are not primarily classified as components of it. The umbilicus, however, stands out as it relates more to the integumentary system and does not have any direct association with the skeletal structure.

Thus, while the pectoral girdle is integral to the skeletal system and its function, the umbilicus represents an area unassociated with the skeletal structure, making it the correct choice in this context.