Weakness of pronator quadratus with AIN entrapment helps differentiate it from carpal tunnel syndrome. Which nerve entrapment is involved in this weakness?

• A. Ulnar nerve entrapment
• B. Median nerve entrapment at the carpal tunnel
• C. Anterior interosseous nerve entrapment
• D. Radial nerve entrapment

Answer: (C)

Weakness of pronator quadratus points to involvement of the anterior interosseous nerve, a pure motor branch of the median nerve in the forearm. The anterior interosseous nerve supplies the deep forearm flexors, including pronator quadratus, flexor pollicis longus, and the lateral half of flexor digitorum profundus. When this nerve is entrapped, those muscles weaken, which helps explain the loss of pronation strength and pinch function. In contrast, carpal tunnel syndrome compresses the median nerve distal to this branching point and typically spares the forearm deep flexors, so pronator quadratus weakness would not be expected. Thus the entrapment involved is the anterior interosseous nerve.