A 35-year-old presents with a lacerating injury to the palm. Examination shows inability to flex the distal phalanx of the index finger and the interphalangeal joint of the thumb, with intact finger flexion at other joints. This indicates injury to:

• A Anterior interosseous nerve (AIN) ✓
• B Recurrent (motor) branch of the median nerve
• C Deep branch of the ulnar nerve
• D Flexor digitorum profundus tendon to index alone

Explanation

The anterior interosseous nerve (AIN), a pure motor branch of the median nerve, innervates flexor pollicis longus (FPL), the lateral half of flexor digitorum profundus (FDP to index and middle fingers), and pronator quadratus. AIN palsy produces the characteristic 'OK sign' failure — inability to make an O-shape, with the distal phalanx of thumb and index finger remaining extended. The recurrent branch (B) innervates thenar muscles (opposition/abduction), not distal flexors. The deep ulnar nerve (C) supplies interossei and medial two lumbricals.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.