Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are:
pulmonary stenosis, which is narrowing of the exit from the right ventricle;
a ventricular septal defect, which is a hole allowing blood to flow between the two ventricles;
right ventricular hypertrophy, which is thickening of the right ventricular muscle; and
an overriding aorta, which is where the aorta expands to allow blood from both ventricles to enter.At birth, children may be asymptomatic or present with many severe symptoms. Later in infancy, there are typically episodes of bluish colour to the skin due to a lack of sufficient oxygenation, known as cyanosis. When affected babies cry or have a bowel movement, they may undergo a "tet spell" where they turn cyanotic, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding.The cause of tetralogy of Fallot is typically not known. Risk factors include a mother who uses alcohol, has diabetes, is over the age of 40, or gets rubella during pregnancy. : 62  It may also be associated with Down syndrome and other chromosomal defects that cause congenital heart defects.TOF is typically treated by open heart surgery in the first year of life. The timing of surgery depends on the babys symptoms and size. The procedure involves increasing the size of the pulmonary valve and pulmonary arteries and repairing the ventricular septal defect.
Primarily, the degree of right ventricular outflow tract obstruction varies between patients and generally determines clinical symptoms and disease progression.Presumably, this arises from an unequal growth of the aorticopulmonary septum (aka pulmonary outflow septum). : 199  The aorta is too large, thus "overriding," and this "steals" from the pulmonary artery, which is therefore stenosed. This then prevents ventricular wall closure, therefore VSD, and this increases the pressures on the right side, and so the R ventricle becomes bigger to handle the work. : 199

Additional anomalies
In addition, tetralogy of Fallot may present with other anatomical anomalies, including:: 66–8 
stenosis of the left pulmonary artery, in 40%
a bicuspid pulmonary valve, in 60%
right-sided aortic arch, in 25%
coronary artery anomalies, in 10%
a patent foramen ovale or atrial septal defect, in which case the syndrome is sometimes called a pentalogy of Fallot
an atrioventricular septal defect
partially or totally anomalous pulmonary venous returnTetralogy of Fallot with pulmonary atresia (pseudotruncus arteriosus) is a severe variant in which there is complete obstruction (atresia) of the right ventricular outflow tract, causing an absence of the pulmonary trunk during embryonic development. : 67–8  In these individuals, blood shunts completely from the right ventricle to the left where it is pumped only through the aorta. The lungs are perfused via extensive collaterals from the systemic arteries, and sometimes also via the ductus arteriosus. : 67–8

Diagnosis
There are three different useful diagnostic tests used for the diagnosis of tetralogy of Fallot. These include a chest radiograph, electrocardiogram, and echocardiogram. The echocardiography determines the final diagnosis and typically offers enough information for surgical treatment planning.