Review Question - QID 220106

A 34-year-old truck driver presents to the office with pain and weakness in his dominant upper extremity after attempting to load a heavy parcel onto his truck. He has ecchymosis along his anterior upper arm extending across the antecubital fossa into his proximal forearm. On exam, there is notable weakness with resisted supination. Hook test demonstrates no palpable tendon. Radiographs and representative images from his MRI are shown in Figures A-C. If he undergoes surgical intervention with a single-incision approach, a branch of which of the sections of the brachial plexus labeled in Figure D is most likely to be injured?