Anterior interosseous nerve (AIN) palsy selectively paralyses which muscles and produces what characteristic clinical sign?

• A Flexor carpi radialis, palmaris longus, flexor digitorum superficialis; weak wrist flexion
• B Flexor pollicis longus, flexor digitorum profundus (index and middle), pronator quadratus; inability to make an 'OK' sign ✓
• C All thenar muscles with spared flexors; thenar wasting without sensory loss
• D Pronator teres and flexor carpi radialis; positive Phalen's test

Explanation

The anterior interosseous nerve (AIN) is a purely motor branch of the median nerve arising just below the elbow, supplying flexor pollicis longus (FPL), the lateral half of flexor digitorum profundus (FDP, to index and middle fingers), and pronator quadratus. AIN palsy is characterised by an inability to flex the terminal phalanges of the thumb and index finger, so the patient cannot make a pinch or 'OK' sign — instead the thumb and index finger form a straight pinch (flat pinch sign). This is distinct from carpal tunnel syndrome which spares FPL and FDP.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.