Final answer:
To prepare a patient with a broken right humerus for transport, the shoulder and elbow joints should be immobilized to ensure stability and minimize further injury.
Step-by-step explanation:
When immobilizing a fracture to prepare for transport, it's important to immobilize the joints above and below the site of the injury. In this case, where a 12 year old has a broken right arm with a humerus fracture, the joints to immobilize are the shoulder and elbow. These joints are crucial for stabilizing the upper arm bone, which has been compromised due to the fracture.
It is essential to consider the potential for vascular and nerve injuries when dealing with a fracture of the humerus. Therefore, immobilizing the shoulder and elbow joints helps to minimize further damage and allows for safe transportation to receive appropriate medical care.