Which joint connects the arm to the torso?

The shoulder joint is the critical connection between the arm and the torso, facilitating a wide range of motion while allowing the upper limb to be attached to the axial skeleton at the sternum through the clavicle. The anatomy of the shoulder joint, particularly the glenohumeral joint, enables increased mobility needed for various activities, from reaching and lifting to throwing.

In contrast, the knee joint serves as the connection between the femur of the thigh and the tibia of the lower leg, facilitating movements related to the lower limb. Similarly, the elbow joint connects the upper arm to the forearm and is primarily responsible for the flexion and extension of the arm, rather than connecting the arm to the torso. The ankle joint links the lower leg to the foot and is implicated in movements related to walking and balance, not in connecting the arm to the trunk. Hence, the shoulder joint remains the correct choice for this particular question.