Which of the following bones is located in the lower arm opposite the thumb?

The ulna is the bone located in the lower arm opposite the thumb. In human anatomy, the forearm consists of two long bones: the radius and the ulna. The ulna runs parallel to the radius but is positioned on the side opposite the thumb, meaning it is on the inner side of the forearm when the arm is in the standard anatomical position with palms facing forward.

This positioning is significant because it influences the movement and rotation of the forearm. The ulna contributes to the structure of the elbow joint and is the larger bone at the wrist end, where it forms a prominent structure known as the ulnar styloid process. Understanding the arrangement and relationship of these bones is crucial for grasping concepts related to arm mechanics and injuries, particularly in relation to activities that involve wrist and forearm movements.

The other choices refer to different bones: the radius is the bone located on the thumb side of the forearm; the scaphoid and lunate are carpal bones in the wrist and do not pertain to the structure of the forearm.