Which motor deficit is commonly seen in carpal tunnel syndrome due to median nerve involvement?

• A. Weakness of thumb opposition/abduction
• B. Weakness of knee extension
• C. Weakness of ankle dorsiflexion
• D. Weakness of shoulder abduction

Answer: (A)

In carpal tunnel syndrome, compression of the median nerve at the wrist mainly affects the muscles of the thenar eminence that the median nerve supplies. The key motor consequence is weakness of thumb opposition and abduction because the opposing and abducting muscles of the thumb (opponens pollicis and abductor pollicis brevis) are innervated by the recurrent branch of the median nerve. When these muscles are weakened, the thumb has trouble moving toward the other fingers (opposition) and lifting away from the hand (abduction), leading to the characteristic functional impairment.

The other options involve nerves that aren’t affected by median nerve compression at the wrist: knee extension relies on the femoral nerve, ankle dorsiflexion on the deep peroneal nerve, and shoulder abduction on the axillary nerve. These would not be expected to weaken in carpal tunnel syndrome.