A young college student injures his left knee while playing football and is unable to bear weight. The provisional x-ray report indicates that there are no fractures seen. He is discharged home but presents the next morning to the emergency department with a severely swollen, painful left knee and severe pain in the foot. On examination, the foot is pale, cold, and pulseless. What is the most likely diagnosis?

Normal radiographic findings in the presence of severe knee trauma should raise suspicion for posterior dislocation of the knee, which is often associated with popliteal artery thrombosis. A careful vascular examination should, therefore, be made in such a situation. The presence of pain, pallor, and pulselessness (three of the five p's) is indicative of severe ischemia. This patient should undergo urgent exploration for vascular repair. The other options are unlikely to cause the signs and symptoms presented.