Final answer:
The most important reason for the client to exercise the affected arm after a modified radical mastectomy is to prevent joint stiffness and maintain range of motion.
Step-by-step explanation:
The most important reason for the client to exercise the affected arm after a modified radical mastectomy is to prevent joint stiffness and maintain range of motion. By exercising the arm, the client can keep the joints flexible and avoid developing stiffness or loss of mobility. This is crucial for daily activities and maintaining independence.