If Debbie broke the small bone on the thumb side of her lower arm, which bone did she break?

The bone that Debbie broke is the radius, which is located on the thumb side of the forearm. The radius and ulna are the two long bones in the forearm; the radius runs parallel to the thumb, while the ulna is positioned parallel to the pinky finger side.

In the context of arm anatomy, when referring to injuries or fractures, identifying the correct bone is crucial for determining the extent of the injury and appropriate treatment. The radius plays a significant role in the movement of the wrist and the hand, particularly in movements like pronation and supination (rotating the forearm).

The other options listed, such as the ulna, carpals, and metacarpals, do not relate to the area described in the question as directly. The ulna is on the opposite side of the forearm and is therefore not the correct answer. Carpals are the bones in the wrist, and metacarpals are the bones in the hand. Understanding the anatomy of these bones helps clarify why the radius is the correct choice in this situation.