NBME Gross Anatomy High Yield Practice Test

Wrist drop is caused by paralysis of the extensor muscles in the posterior forearm, which are supplied by the radial nerve. When the radial nerve is injured—classically with a humeral shaft fracture or compression in the arm—the extensors can no longer extend the wrist and fingers. The hand then rests in a flexed position at the wrist because the flexors, innervated by other nerves, overpower the weakened extensors. The deep branch, or posterior interosseous nerve, specifically innervates most of the finger and wrist extensors, so its disruption produces this characteristic drop. By contrast, injuries to the median, ulnar, or axillary nerves produce other deficits (loss of thumb opposition/thenar function, interossei/claw hand, or deltoid abduction, respectively) and not the classic wrist drop.