Following a mid-shaft fracture of the humerus, which muscle is MOST likely to be weakened due to radial nerve injury?

• A. Flexor digitorum superficialis
• B. Pronator quadratus
• C. Dorsal interossei
• D. Extensor carpi radialis longus

Answer: (D)

When the radial nerve is injured at the mid-shaft of the humerus, it disrupts the muscles that extend the wrist and fingers in the posterior forearm. The triceps may still function because its branches usually come off higher up, so elbow extension is typically preserved. The muscle most affected by this radial-nerve injury at this level is the extensor carpi radialis longus, a key wrist extensor that helps lift the back of the hand and provide wrist stabilization. Weakness here leads to difficulty extending the wrist and some loss of radial deviation, producing a classic wrist-extension deficit pattern after a radial nerve injury.

The other muscles listed are not primarily innervated by the radial nerve at this location: flexor digitorum superficialis is a median-nerve muscle that flexes the fingers; pronator quadratus is median-nerve; and the dorsal interossei are innervated by the ulnar nerve.