What is the main purpose of the MCL (medial collateral ligament)?

The primary function of the medial collateral ligament (MCL) is to provide stability to the knee joint, particularly in resisting forces that may cause the knee to bend inward. This stability is crucial during lateral movements such as cutting, pivoting, or performing side lunges, where the knee is subjected to stresses from the side. The MCL helps to prevent excessive medial movement of the tibia relative to the femur, thus preserving the integrity of the joint during these dynamic actions.

The other options describe functions related to different structures or joints and do not pertain to the MCL. For instance, the stabilization of rotational ankle movements is more relevant to ligaments in the ankle rather than the MCL. Similarly, the stability of the humeral head in the scapular fossa is the function of the shoulder joint, particularly involving the rotator cuff and other shoulder ligaments. The acromio-clavicular joint stability is managed by separate ligaments, such as the acromioclavicular and coracoclavicular ligaments, focusing specifically on the relationships between the clavicle and scapula.