A patient with a deep laceration at the wrist cannot perform the 'O.K.' sign (opposition of thumb tip to index fingertip forming a circle). Which nerve is damaged and at which level?

• A Ulnar nerve at the wrist
• B Median nerve at the wrist
• C Radial nerve at the spiral groove
• D Anterior interosseous nerve (deep branch of median nerve) ✓

Explanation

The 'O.K.' sign requires precise tip-to-tip pinch, which depends on flexor pollicis longus (flexes IP joint of thumb) and flexor digitorum profundus to index finger — both supplied by the anterior interosseous nerve, a branch of the median nerve arising in the forearm. Damage to this purely motor nerve causes inability to flex the distal phalanges of thumb and index, resulting in a pinch sign where the thumb and index meet pulp-to-pulp rather than tip-to-tip. Median nerve damage at the wrist spares FPL and FDP (supplied proximal to wrist).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.