Tetralogy of Fallot is the only cyanotic congenital heart disease on the NCCPA Blueprint. Which congenital heart defect is classically cyanotic?

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Cyanosis in congenital heart disease most often comes from a right-to-left shunt that steals oxygenated blood from the lungs and sends desaturated blood into the systemic circulation. Tetralogy of Fallot is the classic example of this mechanism. The combination of a ventricular septal defect with outflow tract obstruction (pulmonic stenosis) causes reduced flow to the lungs and a tendency for blood to shunt right to left, especially during exertion or stress. The overriding aorta then mixes this desaturated blood into systemic circulation, producing persistent cyanosis and, in many patients, episodic “tet spells” of worsening cyanosis.

Other defects listed can be cyanotic as well, but they operate through different paths and are not the quintessential example used to illustrate the cyanotic physiology in many exam contexts. Transposition of the great arteries involves parallel instead of serial circulation and cyanosis arises from a critical need for mixing at some level; tricuspid atresia and hypoplastic left heart syndrome are also cyanotic due to severe underdevelopment and reliance on shunts for systemic or pulmonary flow. However, Tetralogy of Fallot remains the prototypical cyanotic congenital heart disease that exam questions highlight to exemplify the right-to-left shunt with decreased pulmonary flow.