"Once, I played a practical joke on a sick friend, telling him that I had heard the doctor talking to his parents and that he had only one week left to live. They say humor is the best medicine, but it didn't turn out that way. From that experience, I learned never to make any jokes at all. This shows the serious physician I will be. Indeed, if my patients ever try to joke with me, I will scold them."
You see? We can't help at all without knowing about your personality, sense of humor, and type of physician you hope to become. If you don't feel ready to write a draft of the essay, at least tell us the kinds of things about yourself you might include.