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In anatomical position, the body is standing upright with the arms at the sides and the palms facing forward. In this position, the ulna is located on the medial side of the forearm. This means it is closer to the midline of the body compared to the radius, which is positioned on the lateral side of the forearm.

The ulna is one of the two long bones in the forearm and extends from the elbow to the wrist on the inner side of the arm. It has a significant role in the formation of the elbow joint and provides stability to the forearm. Its structure includes features such as the olecranon and the trochlear notch, which are important for its articulation with the humerus at the elbow.

In contrast, the radius, while also a forearm bone, lies on the outer side (lateral side) in anatomical position. The other two options, the humerus and the sacrum, are not forearm bones; the humerus is the bone of the upper arm, and the sacrum is part of the pelvis. Therefore, identifying the ulna as the bone located on the medial side of the forearm is essential for understanding arm anatomy in relation to the body’s position.