To keep the doctor away, meaning is to avoid any sickness and not going to the doctor to cure it.

That's why a person must consume only an egg a day.

"An egg a day keeps the doctor away"

Based from the slogan above, Why do you think a person must consume only an egg a day?

- Based on the slogan above, i think a person must consume only an egg a day because eating eggs leads to elevated levels of high-density lipoprotein (HDL), also known as the “good” cholesterol. People who have higher HDL levels have a lower risk of heart disease, stroke and other health issues. According to one study, eating two eggs a day for six weeks increased HDL levels by 10%.

Hope it helps...