Final answer:
The client in the scenario is using sublimation, a defense mechanism that transforms negative emotions into positive actions, such as exercising after being accused of stealing.
Step-by-step explanation:
The nurse should recognize that the client is demonstrating the defense mechanism known as sublimation. Sublimation is a defense mechanism where negative or distressing emotions are channeled into positive or socially acceptable activities. In this scenario, the client is redirecting the intense feelings from being accused of stealing into a constructive activity such as going to the gym. This is a mature way of handling stress and negative emotions, as it turns potentially destructive energy into something beneficial.