In Klumpke's palsy, which muscle is most likely weak due to C8-T1 involvement?

• A. Flexor digitorum profundus
• B. Flexor carpi ulnaris
• C. Deltoid
• D. Trapezius

Answer: (B)

Klumpke's palsy involves injury to the lower trunk of the brachial plexus (C8–T1), which disrupts the ulnar nerve distribution. The flexor carpi ulnaris is innervated by the ulnar nerve (C8–T1), so it is the muscle most likely to be weak when this region is affected. Weakness of FCU reduces wrist flexion with ulnar deviation. While the flexor digitorum profundus to the ring and little fingers is also ulnar-innervated and would be weak, FCU provides a more prominent and easily tested sign of lower trunk involvement. The deltoid and trapezius are supplied by nerves (axillary and spinal accessory) that originate from different portions of the plexus or outside it, so they would be spared in a lower trunk injury.