The anterior interosseous nerve is a branch of the median nerve arising just below the elbow. Isolated anterior interosseous nerve palsy produces which characteristic clinical finding?
- A Loss of all thumb movements and thenar atrophy
- B Weakness of pronator teres and loss of sensation over lateral palm
- C Inability to make the 'OK' sign (weakness of FPL and FDP to index finger) ✓
- D Claw hand deformity of all fingers
Correct answer: C. Inability to make the 'OK' sign (weakness of FPL and FDP to index finger)
Explanation
The anterior interosseous nerve (AIN) is a pure motor branch of the median nerve supplying flexor pollicis longus (FPL), the lateral half of flexor digitorum profundus (FDP — index and middle), and pronator quadratus. Because there is no sensory component, AIN palsy produces no sensory loss. The characteristic sign is inability to make the 'OK' sign — when the patient tries to form the 'O', the index finger and thumb remain straight (hyperextended) due to paralysis of FPL and FDP to index finger. This differentiates it from carpal tunnel syndrome which has sensory symptoms.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.