A doctor would prescribe medicine to someone who is in good health primarily to support specific physiological needs or prevent future health issues. In the given responses, the best example would be:
Your mother needs special vitamins to keep the developing baby inside growing healthy.
In this case, even though the mother may be in good health herself, the doctor prescribes prenatal vitamins to ensure both the mother and the developing baby receive the necessary nutrients for a healthy pregnancy. This proactive approach is crucial for preventing deficiencies that could affect the health of both the mother and the fetus.