An 18-year-old college football player comes to your office after sustaining an injury on the field. He states that he planted his foot and had to pivot to catch the ball. He heard a "pop" and his right knee gave way and started to swell. The trainer iced it down. On physical examination you note increased swelling and tenderness over the right knee. He has significant forward excursion when you perform the Lachman test. He also has more movement of the right tibia when you draw it forward when compared with the left. Based on this information what is your most likely diagnosis