Final answer:
The patient is using the defense mechanism of rationalization, justifying their leniency with their diet by emphasizing the exercise they are doing.
Step-by-step explanation:
The defense mechanism the medical assistant should identify that the patient is using is rationalization. In this context, rationalization is the attempt to justify or make excuses for one's behavior, in this case, the client's inadequate adherence to dietary recommendations for managing heart disease. The client acknowledges not being careful about salt and fat intake but attempts to mitigate the significance of this by emphasizing an increase in physical activity, which they perceive as a positive behavior. By focusing on the exercise, the client might be trying to rationalize the less vigilant approach to diet as less detrimental or significant.