When asked to hold a sheet of paper between the thumb and index finger, a patient flexes the interphalangeal joint of the thumb to maintain grip. Which muscle is paralysed and which nerve is at fault?

• A Flexor pollicis brevis, median nerve
• B First dorsal interosseous, radial nerve
• C Adductor pollicis, ulnar nerve ✓
• D Flexor pollicis longus, anterior interosseous nerve

Explanation

This is Froment's sign. Adductor pollicis, supplied by the deep branch of the ulnar nerve, normally holds the thumb against the index finger in the plane of the palm. When it is paralysed, flexor pollicis longus, supplied by the median nerve, compensates by flexing the thumb at the interphalangeal joint. In option D, flexor pollicis longus itself would be paralysed, so this compensation could not occur.

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

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Written and medically reviewed by the StethoPrep medical team.