In children with tetralogy of Fallot,why does unoxygenated blood enter the systemic circulation?

Question

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In children with tetralogy of Fallot, there is usually pulmonary stenosis, which causes an increase in pressure in the right ventricle compared to the left ventricle. As a result, when blood is pumped out of the right ventricle, some of the unoxygenated blood present in the right ventricle is able to mix with the oxygenated blood from the left ventricle due to the pressure difference between the two ventricular chambers. This mixed blood then enters the systemic circulation.

In Children with Tetralogy of Fallot,why Does Unoxygenated Blood Enter