A 35-year-old woman presents with inability to flex the terminal phalanx of the thumb and index finger. She cannot form an 'OK sign.' MRI shows a ganglion cyst at the elbow compressing a nerve. Which nerve and which branch is involved?
- A Median nerve — anterior interosseous nerve (AIN) ✓
- B Ulnar nerve — deep branch
- C Radial nerve — posterior interosseous nerve (PIN)
- D Median nerve — recurrent (thenar) branch
Correct answer: A. Median nerve — anterior interosseous nerve (AIN)
Explanation
The 'OK sign' test assesses flexor pollicis longus (FPL) and flexor digitorum profundus to the index (FDP-index), both innervated by the anterior interosseous nerve (AIN), a pure motor branch of the median nerve. Inability to form the OK sign (terminal pinch replaced by pulp-to-pulp contact) is pathognomonic of AIN syndrome. The AIN arises in the proximal forearm and can be compressed by a fibrous band, Gantzer's muscle, or a ganglion cyst. PIN compression (radial nerve) causes wrist/finger drop, not AIN pattern.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.