Review Question - QID 214109

A 25-year-old right-hand dominant painter presents with isolated right elbow pain after a 5-foot fall from a ladder. On exam, he is neurovascularly intact with tenderness and swelling about the elbow. Apprehension and pain are noted when flexing the supinated elbow from an extended position with simultaneous axial and valgus stress. Imaging demonstrates reduced radiocapitellar and ulnohumeral joints with a defect noted on the posterolateral capitellum. Disruption of which of the following is responsible for the findings on physical exam.