Review Question - QID 8596

A high school athlete sustained a noncontact injury to his right knee. He says that during a football game he felt a pop and his leg gave way. He attempted to continue to play but was unable secondary to pain. Five days after the injury, radiographs of his right knee do not reveal any abnormalities. On examination, he has an effusion on the injured side and no joint line tenderness. His range of motion is full extension to 110 degrees of flexion. At 20 degrees of flexion, he has increased anterior translation compared to the contralateral, uninjured left side. At 90 degrees of flexion, the tibia does not translate posteriorly. As his knee is moved from full extension into flexion with an internal rotation and valgus force, you notice a "clunk" within the knee. What is the most likely biomechanical basis for the "clunk"?