A patient can't lift the arm sideways after a shoulder injury, and the outer shoulder is numb. This points to the axillary nerve (C5–C6) — it winds around the surgical neck of the humerus, supplying the deltoid (arm abduction) and skin over the shoulder. Fractures of the surgical neck commonly injure it. The deltoid's loss explains the weakness; the numb patch confirms the nerve, not just the muscle, is hurt.