A 19-yeаr-оld sоccer аthlete limps оff the field in pаin. He tells you he pivoted abruptly and felt a sharp severe pain in his right knee. He cannot bear weight on it and swelling is noted. On examination, you find right knee effusion, medial joint-line tenderness, he is unable to deep knee bend and has a positive McMurray sign. States he feels a “locking” sensation when he tries to walk. The H&P findings are suggestive that he may have: