What is the most important subject does a doctor have to study?

There's no single "most important" subject for a doctor to study, as medical practice requires a broad and deep understanding of many interconnected areas. However, if forced to choose one overarching category, it would be human biology and its pathologies. This encompasses:

* Anatomy: Understanding the structure of the human body.

* Physiology: Understanding how the body functions normally.

* Pathophysiology: Understanding how disease processes disrupt normal bodily functions.

* Biochemistry: Understanding the chemical processes within the body and how they are affected by disease.

* Genetics: Understanding the role of genes in health and disease.

* Microbiology: Understanding microorganisms and their role in infection.

While other subjects like pharmacology, surgery, diagnostics, and ethics are crucial, a strong foundation in human biology and its pathologies underpins the ability to diagnose, treat, and prevent disease. All other medical knowledge builds upon this fundamental understanding.