A patient presents with weakness in wrist ulnar deviation. Which muscle may be affected?

The muscle affected in a case of weakness in wrist ulnar deviation is the flexor carpi ulnaris. This muscle is primarily responsible for bending (flexing) the wrist as well as facilitating ulnar deviation, which is the movement of turning the wrist towards the ulna (the inner side of the forearm).

When the flexor carpi ulnaris is compromised, the ability to perform ulnar deviation weakens significantly, leading to difficulties in that specific movement. The other muscles listed do not contribute to ulnar deviation as effectively. For example, the palmaris longus mainly aids in wrist flexion without a strong emphasis on ulnar deviation. The flexor carpi radialis, while it contributes to wrist flexion, works more towards radial deviation (movement towards the thumb side) rather than ulnar deviation. Lastly, the extensor carpi ulnaris assists in extending the wrist and ulnar deviation but is not as pivotal for this action as the flexor carpi ulnaris is in the context of wrist flexion alongside ulnar movement. Thus, weakness in wrist ulnar deviation indicates a potential issue with the flexor carpi ulnaris.