You should clarify your question. Do you mean what do doctors need to study after becoming a doctor, and/or what do they study in med school, and/or what they study before med school? All of this also depends on your global region.
A vetrinarian
A cardiologist studies and treats the human heart. Another name for a heart doctor is a cardiologist.
The kind of scientist who studies Urology would be a Biologist, since Urology concerns a biological function (urination). If you mean the kind of doctor who studies Urology, that would be a Urologist.
A doctor who specializes in heart problems is called a cardiologist.
A doctor that studies hormones is an endocrinologist.
A rheumatologist is a doctor who studies joints.
An Epidemiologist studies the spread of disease.
a masters degree and lots of medical classes
7 to 8 years depends on what kind of doctor you want to be also. - Amel Kharchenko
jurisprudence
An entomologist is a scientist that studies insects and 'bugs.' A chiropterologist is a scientist who studies bats. An ornithologist studies birds. It would depend on what small, winged creature was being studied.
You need to answer that question your self. Know the kind of person you are and answer the question.