What part of the humerus serves as the attachment site for the subscapularis tendon?

The lesser tuberosity of the humerus is the correct answer because it is specifically designated as the attachment site for the subscapularis tendon. The subscapularis muscle is one of the four rotator cuff muscles and plays a crucial role in the internal rotation of the arm. Its tendon attaches firmly to the lesser tuberosity, allowing for effective movement and stabilization of the shoulder joint.

In contrast, the greater tuberosity serves as an attachment point for other rotator cuff muscles, specifically the supraspinatus, infraspinatus, and teres minor muscles. The medial epicondyle and trochlea are anatomical landmarks on the humerus, but they do not serve as attachment sites for the subscapularis tendon. The medial epicondyle is primarily associated with the flexor muscles of the forearm, while the trochlea is involved in the articulation of the humerus with the ulna. This anatomical specificity highlights the importance of each structure in providing a functional role in shoulder mechanics.