A long-distance cyclist develops weakness of all finger interossei and hypothenar muscles with numbness over the little finger pulp, but sensation on the dorsum of the hand is completely normal. The lesion is localised to:

• A The ulnar nerve at the cubital tunnel behind the medial epicondyle
• B The deep branch of the ulnar nerve at the pisohamate hiatus with palmar sensory involvement
• C The lower trunk of the brachial plexus at the thoracic outlet
• D The ulnar nerve within Guyon canal, distal to the origin of the dorsal cutaneous branch ✓

Explanation

The dorsal cutaneous branch of the ulnar nerve arises in the forearm, well proximal to the wrist, and supplies the dorsum of the hand. Preserved dorsal sensation with motor loss and palmar sensory loss localises the lesion to Guyon canal, classically caused by compression against the hook of hamate in cyclists. A cubital tunnel or thoracic outlet lesion would involve the dorsal cutaneous branch and abolish dorsal sensation. Option B is self contradictory since the deep branch carries no sensory fibres.

Reference: Green's Operative Hand Surgery, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.