Give a stream of consciousness and then the final answer. The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist valgus forces on the knee.

Answer this question: what is the function of the medial collateral ligament?
To answer the above question, the relevant sentence is: Its primary function is to resist valgus forces on the knee.
So, the final answer is to resist valgus forces on the knee.