MSFinals-4027

An 8-year-old girl is brought to the Emergency Department by her parents after she fell off a swing. She reports that she was able to brace her fall with her left hand. In the Emergency Department, the patient’s vitals are stable. Examination of the left upper extremity demonstrates tenderness above the elbow, with evidence of soft tissue swelling. A plain film demonstrates a supracondylar fracture of the left humerus.
Which of the following structures is most at risk?