A 55-year-old man presents with inability to actively pronate the forearm and weakness of wrist and finger flexors with preserved intrinsic hand muscles, 4 months after a distal biceps tendon repair surgery. Nerve conduction shows absent compound muscle action potential from the anterior interosseous nerve territory. The most likely nerve injury is:
- A Anterior interosseous nerve (AIN) palsy ✓
- B Median nerve at the wrist (carpal tunnel syndrome)
- C Posterior interosseous nerve (PIN) palsy
- D Ulnar nerve at the elbow (cubital tunnel syndrome)
Explanation
Anterior interosseous nerve (AIN) palsy presents as inability to form a pinch ('OK sign' test fails — cannot flex terminal phalanx of thumb and index finger): paralysis of FPL, FDP to index/middle fingers, and pronator quadratus. The 'pinch posture' is extension of both IP joints. In this question, forearm pronation weakness (pronator quadratus) and wrist/finger flexor weakness with preserved intrinsics fits AIN territory (purely motor, no sensory loss). AIN is a branch of median nerve arising at the cubital fossa level; it can be injured by distal biceps surgery. PIN palsy affects wrist/finger extension (extensor compartment). Median nerve at wrist (CTS) causes sensory symptoms + thenar atrophy.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.