What motor function is primarily affected by a proximal median nerve injury in the hand?

• A. Thumb opposition and abduction (thenar muscle function)
• B. Wrist extension
• C. Finger flexion
• D. Little finger abduction

Answer: (A)

Loss of thumb opposition and abduction is the key motor deficit because the median nerve’s recurrent branch in the hand innervates the thenar muscles (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis). These muscles are responsible for bringing the thumb toward the other fingers (opposition) and moving it away from the palm (abduction). When a proximal median nerve injury disrupts this innervation, the thenar muscles weaken or atrophy, so the thumb can no longer oppose or abduct effectively. This change is a hallmark of median nerve injury in the hand and underlies the characteristic difficulty with precision pinch, often described as ape hand deformity due to unopposed adductor pollicis (ulnar nerve). The other options involve muscles served by other nerves or functions not primarily affected by this lesion (for example, wrist extension by the radial nerve or little finger abduction by the ulnar nerve).