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?