Which congenital heart disease is classically associated with cyanosis due to a right-to-left shunt?

• A. Tetralogy of Fallot
• B. Transposition of the Great Arteries
• C. Ventricular Septal Defect
• D. Atrial Septal Defect

Answer: (A)

Cyanosis from a right-to-left shunt is most classically seen with Tetralogy of Fallot. In this condition, an underlying ventricular septal defect allows mixing, but the standout feature is obstruction of the right ventricular outflow tract (pulmonary stenosis). The outflow obstruction raises pressures on the right side, promoting deoxygenated blood to cross the VSD from right to left and enter the aorta, bypassing the lungs. This reduces systemic oxygenation and produces cyanosis; it’s often accompanied by spells and the tendency to squat to increase systemic vascular resistance and lessen the shunt.

Other defects don’t fit as neatly. Transposition of the great arteries involves two parallel circulations that require mixing at a shunt like a patent foramen ovale or VSD for survival, so cyanosis stems from the overall circuit arrangement rather than a classic right-to-left VSD shunt. Ventricular septal defect by itself mainly causes a left-to-right shunt and cyanosis only if Eisenmenger physiology develops later. Atrial septal defect likewise is typically left-to-right with cyanosis not expected until late reversal occurs.