A patient with an ulnar nerve lesion is asked to grasp a sheet of paper between the thumb and the radial side of the index finger. The thumb IP joint visibly flexes as he squeezes. This sign indicates paralysis of which muscle?
- A Flexor pollicis brevis
- B Adductor pollicis ✓
- C Opponens pollicis
- D First dorsal interosseous
Correct answer: B. Adductor pollicis
Explanation
Froment's sign is positive when adductor pollicis, an ulnar-innervated muscle, is paralysed and flexor pollicis longus, supplied by the anterior interosseous branch of the median nerve, substitutes by flexing the thumb IP joint to hold the paper. Opponens pollicis and flexor pollicis brevis are thenar muscles tested by opposition, not by paper grip.
Reference: Snell's Clinical Anatomy by Regions, 10th ed.
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