Froment's sign indicates weakness of which muscle?

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

Answer: (C)

Froment's sign shows weakness of the thumb's adductor muscle, the adductor pollicis, which is innervated by the deep branch of the ulnar nerve. In a normal pinch, the adductor pollicis steadies and brings the thumb toward the index finger to create a strong grip. When this muscle is weak, the person substitutes by flexing the thumb’s interphalangeal joint using the flexor pollicis longus to hold the paper, producing a visible IP joint flexion. This compensatory flexion is characteristic of a positive Froment's sign and points to ulnar nerve involvement affecting the adductor pollicis. The other muscles listed are median-nerve–innervated and their weakness would not produce this compensatory pattern.