The medical profession is very specific and belongs to the most knowledge-intensive and responsible spheres of human professional activity.
That's why you have to study for 6 years to become a doctor, and after that, you also have to complete an internship. In addition, medical universities do not offer correspondence courses. It is very difficult to study because of the large amount of factual material, which, often, simply has to be memorized.
There is a lot to learn. Studying can take up all of your time.
For me, the important things in medicine are open wounds, virology, surgery, infections, and so on.
That's what you need to study if you want to be a great doctor.