Anterior interosseous nerve (AIN) syndrome specifically involves which combination of muscles?

• A Flexor digitorum superficialis, flexor carpi radialis, pronator teres
• B Flexor pollicis longus, lateral half of flexor digitorum profundus, pronator quadratus ✓
• C All lumbricals of the hand and thenar muscles
• D Flexor carpi ulnaris and medial half of flexor digitorum profundus

Explanation

The anterior interosseous nerve is a pure motor branch of the median nerve, arising 4-6 cm below the lateral epicondyle. AIN syndrome causes paralysis of flexor pollicis longus (loss of IP joint flexion of thumb), lateral two slips of flexor digitorum profundus (loss of DIP flexion of index and middle fingers), and pronator quadratus (weak pronation). Classic sign is inability to make the 'OK' sign — patient forms a pinch with extension of both IP joints instead of flexion (triangular pinch). There is no sensory deficit because AIN is purely motor.

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

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Written and medically reviewed by the StethoPrep medical team.