Final answer:
The most beneficial lifestyle modification for the patient is engaging in regular aerobic exercise, which can help reduce the risk of cardiovascular diseases linked to her family history.
Step-by-step explanation:
The most important lifestyle modification for a 42-year-old patient with a family history of hypertension, hypercholesterolemia, and diabetes mellitus, and who has a normal BMI and no current health issues, would be starting an aerobic exercise program. Regular aerobic activities help keep the heart healthy and reduce the risk of cardiovascular diseases. To minimize the risk associated with her family history, engaging in at least 60 minutes of aerobic exercise five days a week is recommended. Other healthy behaviors such as maintaining a balanced diet low in salt and saturated fats, and periodic health screenings to monitor blood pressure and cholesterol levels can also be beneficial.