Which of the following is considered the least radiosensitive?

The least radiosensitive group among the options provided is the adult patient. This is because adults have more mature and differentiated cells compared to children and fetuses, which makes them generally less susceptible to the harmful effects of ionizing radiation.

In younger patients, such as teenagers or pediatric patients, and in fetuses, cells are still rapidly dividing and are more vulnerable to DNA damage caused by radiation exposure. Rapidly dividing cells have a higher likelihood of being adversely affected when exposed to radiation, leading to greater radiation sensitivity.

Furthermore, the fetus is particularly sensitive because it is at a critical stage of development, where any damage can have severe implications on development and health. Teenage and pediatric patients also fall into a more radiosensitive category due to their active growth and development phases.

Thus, the physiological factors related to the stage of development and the rate of cell division significantly contribute to why adult patients exhibit the least radiosensitivity compared to the other groups listed.