The anterior interosseous nerve (AIN) is a branch of the median nerve that supplies no cutaneous area but innervates specific muscles. Which motor deficit best identifies an isolated AIN palsy?

• A Wrist drop with inability to extend the fingers
• B Loss of thumb opposition and thenar wasting
• C Inability to flex the terminal phalanx of index finger and thumb (failure of OK sign/pinch) ✓
• D Inability to extend the index finger with preserved wrist extension

Explanation

The anterior interosseous nerve branches from the median nerve 5–8 cm below the lateral epicondyle and innervates flexor pollicis longus (FPL), the lateral half of flexor digitorum profundus (FDP to index and middle fingers), and pronator quadratus. AIN palsy results in loss of FPL and FDP to the index finger, so the patient cannot flex the terminal phalanges of the thumb and index finger — they cannot make the 'OK' sign and instead make a pinch with their more proximal joints (square pinch sign). There is no sensory loss as AIN has no cutaneous distribution. Wrist drop is radial nerve palsy. Thenar wasting is recurrent motor branch of median nerve or carpal tunnel syndrome.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.