Final answer:
The Medial Collateral Ligament (MCL), or the ulnar collateral ligament, would most likely be overstretched from a valgus force to the lateral aspect of the elbow, as it resists such forces in the medial elbow structures.
Step-by-step explanation:
The ligament that would MOST LIKELY sustain an overstretch injury from a valgus force to the lateral aspect of the elbow is the Medial Collateral Ligament (MCL), also known as the ulnar collateral ligament. This intrinsic ligament is found on the medial side of the elbow joint spanning from the medial epicondyle of the humerus to the medial side of the proximal ulna. When a valgus force is applied, it stresses the medial structures of the elbow, hence affecting the MCL. This ulnar collateral ligament resists excessive valgus forces, which commonly occur in overhead throwing activities, and is the same ligament repaired during Tommy John surgery.