What is the distal attachment site of the flexor carpi ulnaris?

The flexor carpi ulnaris muscle is primarily responsible for flexing and adducting the wrist. Its distal attachment sites are critical for its function. The muscle originates from the medial epicondyle of the humerus and inserts into specific bones in the wrist and hand.

The correct choice identifies the pisiform bone, the hook of the hamate, and the base of the 5th metacarpal as the distal attachment points. The pisiform serves as a sesamoid bone that provides a connection to the flexor carpi ulnaris, while the hook of the hamate is also essential for the muscle's functionality, providing leverage and stability. The attachment to the 5th metacarpal specifically aids in wrist adduction and flexion, allowing for effective grip and hand movement.

This accurate identification of attachment sites highlights the muscle's role in facilitating movements of the wrist and hand. The other options include incorrect combinations of bones that do not fully represent the actual insertion points of the flexor carpi ulnaris, thus reinforcing why option B is the correct answer.