a 45-year-old woman presents to your office with right knee pain. she was playing volleyball yesterday when she collided with another player and was unable to continue playing. the knee was swollen this morning. she is able to walk but it is painful, and she also has pain when she attempts to bend her knee. on examination you have the patient stand flat footed on her right leg while you provide your shoulder for balance. you have her then flex her right knee to 20 degrees and rotate the femur on the tibia medially and laterally. this motion causes significant pain for the patient. you also note medial joint line tenderness. which one of the following is the most likely cause of her knee pain?