The outer bone of the forearm, on the side of the thumb.

• A. Ulna
• B. Proximal
• C. Patella
• D. Radius

Answer: (D)

The main idea is identifying which forearm bone is on the thumb side in the standard position. The bone on the outer side of the forearm, toward the thumb, is the radius. In the usual anatomical position, the palm faces forward and the thumb points to the side, so the radius sits on the lateral (thumb) side while the ulna is on the medial (pinky) side. The radius and ulna run parallel and allow forearm rotation—pronation and supination—by the radius rotating around the ulna. The radius also forms part of the wrist joint with the carpal bones. The other options aren’t bones or don’t indicate the correct bone: one is a directional term, and the other is a kneecap.