Which of the following is a cyanotic congenital heart defect?

• A. Coarctation of the aorta
• B. Tetralogy of Fallot
• C. Atrial septal defect
• D. Ventricular septal defect

Answer: (B)

Cyanosis in congenital heart disease happens when deoxygenated blood enters the systemic circulation, usually from a right-to-left shunt or from severely reduced blood flow to the lungs. Tetralogy of Fallot causes cyanosis because the pulmonary stenosis (narrowed outflow to the lungs) decreases lung blood flow, and with an accompanying ventricular septal defect, deoxygenated blood from the right ventricle can cross the VSD and enter the aorta. This right-to-left mixture lowers arterial oxygen levels, producing the characteristic blue tint.

The other defects listed are typically acyanotic early in life. Coarctation of the aorta is an obstruction of the aorta that mainly affects systemic blood flow and pulses rather than causing a right-to-left shunt; cyanosis is not the defining feature here. Atrial septal defect and ventricular septal defect usually create left-to-right shunts, increasing pulmonary blood flow rather than decreasing it; cyanosis would not be expected unless a long-term reversal of the shunt (Eisenmenger physiology) develops later in life, which is not the usual initial presentation.

So the defect that classically presents with cyanosis in infancy is Tetralogy of Fallot due to its combination of decreased pulmonary blood flow and a right-to-left shunt.