Review Question - QID 214989

A 32-year-old man presents to the emergency department with a 1 hour history of pain in his left hand after he fell from a ladder. He says that he tried to break his fall with his left arm and immediately felt pain in the extremity after landing. He has no significant past medical history, and his prior orthopedic history includes a right tibial fracture as a child. On presentation, he is found to have abrasions and ecchymosis over the left distal forearm and left hand. Anteroposterior and lateral radiographs of the left wrist are obtained, and the results are shown in Figure A. Which of the following is most likely responsible for this patient's symptoms?