Tetralogy of Fallot is a congenital heart defect characterized by a combination of four anatomical abnormalities in the heart. These abnormalities include ventricular septal defect (VSD), pulmonary stenosis, overriding aorta, and right ventricular hypertrophy. This condition results in inadequate oxygenation of blood, leading to cyanosis (bluish color of the skin) and various other complications.