Tetralogy of Fallot is a condition caused by a combination of four heart defects that are present at birth:

• Ventricular septal defect or VSD- is a hole in the septum (or wall) between the heart’s two lower chambers – the ventricles.
• Pulmonary stenosis – is when the valve which allows blood to flow from the heart to the lungs –the pulmonary valve – cannot fully open.
• Right ventricular hypertrophy- is when the muscle of the main pumping chamber on the right side – the right ventricle – is thicker than usual.
• Overriding aorta- the main blood vessel that carries blood away from the heart – the aorta – is an overriding aorta when it is located in the wrong place, specifically when it is between the left and right ventricles, directly over the VSD.