A 25-year-old sustains a deep laceration at the wrist and is unable to flex the interphalangeal joint of the thumb (FPL) and the distal phalanx of the index and middle fingers (FDP to index). Which nerve is injured and at which fascicular zone?

• A Median nerve at the elbow affecting anterior interosseous nerve branch
• B Anterior interosseous nerve (AIN) palsy at the forearm — spares sensation, affects FPL and FDP to index/long ✓
• C Median nerve at the wrist (carpal tunnel) — sensory only at this level
• D Ulnar nerve at Guyon's canal

Explanation

The anterior interosseous nerve (AIN) is a purely motor branch of the median nerve arising in the proximal forearm. It innervates FPL, FDP to index and long fingers, and pronator quadratus. AIN palsy produces the characteristic 'no OK sign': the patient cannot form a circle with thumb and index because IP joint of thumb (FPL) and DIP joint of index (FDP) cannot flex — they remain in extension, producing a pinch with extended rather than flexed tips. Sensation is preserved (purely motor nerve). Median nerve at wrist (carpal tunnel) primarily affects sensation and thenar muscles, not FPL/FDP.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.