Froment's sign indicates weakness of which muscle due to ulnar nerve palsy?

• A. Adductor pollicis
• B. Abductor pollicis brevis
• C. Flexor pollicis longus
• D. Opponens pollicis

Answer: (A)

Froment's sign tests the thumb’s ability to adduct, a key function of the adductor pollicis. This muscle is innervated by the deep branch of the ulnar nerve and provides the force to hold the thumb against the index finger when making a pinch. When the ulnar nerve is palsied and the adductor pollicis is weak, the grip cannot be maintained through adduction. To compensate, the person flexes the thumb’s interphalangeal joint via the flexor pollicis longus (which is median-nerve–innervated) to grip the paper. This compensatory IP joint flexion during a pinch is the Froment sign.

The other muscles listed are primarily innervated by the median nerve and do not produce this compensatory pattern when weak, so they don’t explain Froment’s sign in ulnar nerve palsy.