Tetralogy of Fallot (ToF) is a congenital heart defect that is classically understood to involve four anatomical abnormalities of the heart. These abnormalities are:

1. Ventricular Septal Defect (VSD): This is a hole between the two bottom chambers of the heart (the ventricles). It allows oxygen-poor blood to mix with oxygen-rich blood, which then reduces the amount of oxygen-rich blood circulating in the body.
2. Pulmonary Stenosis: This is a narrowing of the pulmonary valve and outflow tract, which leads from the right ventricle to the pulmonary artery. It limits the flow of blood from the heart to the lungs, making it harder for the heart to pump blood to the lungs.
3. Right Ventricular Hypertrophy: This is a thickening of the muscular walls of the right ventricle, which occurs because the right ventricle has to work harder to pump blood through the narrow pulmonary valve.
4. Overriding Aorta: This refers to the aorta (the main artery that carries oxygen-rich blood from the heart to the body) being positioned directly over the ventricular septal defect, rather than stemming only from the left ventricle. As a result, oxygen-poor blood from the right ventricle can flow directly into the aorta.

Individuals with ToF typically have a characteristic "blue" skin coloration (cyanosis) due to the reduced oxygen levels in their blood. Treatment for ToF usually involves surgical intervention to repair the heart defects, often performed in infancy or early childhood. Advances in surgical techniques and postoperative care have greatly improved the outcomes for those with ToF, allowing many to lead relatively normal lives. However, long-term follow-up and sometimes additional surgeries may be necessary as the person ages.