Ans. is 'c' i.e., TOF Tetralogy of fallot is the commonest cyanotic congenital hea disease. Cyanotic Congenitial hea diseases o Cyanotic CHDs are Right to Left shunts. o These are fuher divided into : ? 1) Cyanotic CHD with decreased pulmonary blood flow. This group includes TOF, Pulmonary atresia with intact septum, tricuspid atresia, total anomalous pulmonary venous return with obstruction. These lesions have following components : - a)Obstruction to pulmonary blood flow at tricuspid right ventricular or pulmonary valve level. b)A pathway by which systemic venous blood enters the systemic circulation a patent foramen ovale or ASD or VSD. Degree of cyanosis depends on the degree of obstruction to pulmonary blood flow : - i) Mild obstruction Cyanosis is precipitaed by stress, but may be absent at rest. ii) Severe obstruction Pulmonary blood flow is dependent on patency of the ductus aeriosus. When the ductus closes (10-21 days), the neonate experience profound hypoxemia, cyanosis and shock. 2) Cyanotic CHD with increased pulmonary blood flow. This group of lesions is not associated with obstruction to pulmonary blood flow. Cyanosis caused by any of the following mechanisms. i) Abnormal ventricular-aerial connection (e.g., Transposition of great vessels) In this, aoa arises from ventricle, So that systemic venous blood returning to the right atrium is pumped directly back to the body, and oxygenated blood returning from lung is pumped back into the lungs. ii) Total mixing of systemic venous and pulmonary venous blood (e.g., total anomalous pulmonary venous return, truncus aeriosus, a common atrium or ventricle) Deoxygenated systemic venous blood and oxygenated pulmonary venous blood mix completely in the hea and, as a result, oxygen saturation is equal in the pulmonary aery and aoa. If pulmonary blood flow is not obstructed, these infants have a combination of Cyanosis and hea failure. In Contrast, if pulmonary stenosis is present, these infants have cyanosis alone.