Final answer:
The tibial collateral ligament is responsible for preventing the internal rotation of the lower leg, aiding in the stability of the knee alongside the anterior and posterior cruciate ligaments.
Step-by-step explanation:
The ligament responsible for preventing the internal rotation of the lower leg is the tibial collateral ligament. This ligament, located on the medial side of the knee, is considered an extrinsic ligament of the knee joint and is vital in maintaining knee stability. It spans from the medial epicondyle of the femur to the medial tibia, and alongside the fibular collateral ligament on the lateral side, works to resist both hyperextension and rotation of the extended knee joint.
Meanwhile, the two intracapsular ligaments, anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), contribute to the overall stability of the knee, with the PCL particularly preventing anterior displacement of the femur when the knee is flexed and weight bearing.