A 17-year-old boy from Calcutta was playing football when he was tackled by another player. The opponent hit the lateral aspect of his knee. He presented to a hospital 1 week after the injury, complaining of swelling and pain in his right knee. On physical examination of his right knee, there is a large effusion. There is increased laxity (as compared to his uninjured knee) of his knee when his knee is passively placed in a valgus (abducted) position. In addition, there is significant anterior translation of his tibia with respect to his femur when his knee is tested at ninety degrees of flexion. Injury to which of the following structures most likely accounts for the increased laxity of his knee when his knee is passively placed in a valgus position?

The medial collateral ligament originates from the medial femoral epicondyle and inses on the medial proximal tibia. Its function is to prevent valgus instability of the knee. When a valgus stress is placed on the tibia, the medial collateral ligament is stressed and prevents displacement. The anterior cruciate ligament passes from the medial surface of the lateral femoral condyle to the anterior intercondylar area of the tibia. Its primary function is to prevent anterior translation of the tibia in reference to the femur (or posterior displacement of the femur in reference to the tibia). The lateral collateral ligament originates on the lateral femoral epicondyle and inses on the head of the fibula. It functions to prevent varus instability of the knee. When a varus stress (adduction) is placed on the tibia, the lateral collateral ligament becomes taut and prevents displacement. The patellar ligament runs from the inferior pole of the patella to the tibial tubercle, and is commonly referred to as the patellar tendon. However, this structure connects bone to bone, which is the definition of a ligament, not a tendon.