Weakness of flexor pollicis longus with lateral FDP weakness points to entrapment of which nerve?

• A. Ulnar nerve entrapment
• B. Median nerve entrapment
• C. Anterior interosseous nerve entrapment
• D. Radial nerve entrapment

Answer: (C)

This pattern points to involvement of the anterior interosseous nerve, a motor branch of the median nerve in the forearm. The anterior interosseous nerve supplies the flexor pollicis longus and the lateral (index finger) half of the flexor digitorum profundus, as well as the pronator quadratus. When this nerve is entrapped, those muscles weaken, so you lose tip-to-tip pinch strength between the thumb and index finger, producing difficulty with flexing the thumb’s distal phalanx and the index finger’s DIP joint. Clinically, this manifests as an inability to pinch to form an “OK” sign, with no sensory loss since this nerve is motor-only. This distinguishes anterior interosseous entrapment from other nerve problems, such as ulnar nerve entrapment (affects intrinsic hand muscles and the medial FDP) or radial nerve entrapment (affects extensors).