After a deep laceration of the proximal right forearm, a patient cannot extend the metacarpophalangeal joints of the fingers, but wrist extension is preserved with radial deviation. There is no sensory loss anywhere. Where is the lesion?

• A Radial nerve in the spiral groove
• B Superficial branch of the radial nerve
• C Posterior interosseous nerve within the supinator ✓
• D Median nerve at the elbow

Explanation

The posterior interosseous nerve is purely motor. Branches to brachioradialis, ECRL and ECRL-type wrist extensors are given off before it enters supinator through the arcade of Frohse, so wrist extension survives with radial deviation while finger extension fails. A spiral groove lesion would abolish triceps and cause sensory loss over the dorsum of the hand, excluding option A.

Reference: Snell's Clinical Anatomy by Regions, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.