<s>[INST] context: Tetralogy of fallot is the commonest cyanotic congenital heart disease.
Cyanotic Congenitial heart diseases 

Cyanotic CHDs are Right to Left shunts.
These are further 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 via a patent foramen ovale or ASD or VSD.

The degree of cyanosis depends on the degree of obstruction to pulmonary blood flow : -


Mild obstruction → Cyanosis is precipitaed by stress, but may be absent at rest.
Severe obstruction → Pulmonary blood flow is dependent on patency of the ductus arteriosus. When the ductus closes (10-21 days), the neonate experience profound hypoxemia, cyanosis and shock.

2) Cyanotic CBD 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.



Abnormal ventricular-arterial connection (e.g., Transposition of great vessels) → In this, aorta arises from rt 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.
Total mixing of systemic venous and pulmonary venous blood (e.g., total anomalous pulmonary venous return, truncus arteriosus, a common atrium or ventricle) → Deoxygenated systemic venous blood and oxygenated pulmonary venous blood mix completely in the heart and, as a result, oxygen saturation is equal in the pulmonary artery and aorta. lf pulmonary blood flow is not obstructed, these infants have a combination of Cyanosis and heart failure. In Contrast, if pulmonary stenosis is present, these infants have cyanosis alone. [/INST] question: The commonest type of song. cyanotic heart disease is - | options: A: ASD B: VSD C: TOF D: PDA | answer: C </s>