A 4-month-old male infant presents with progressive cyanosis since 6 weeks of age and a harsh systolic ejection murmur best heard at the left upper sternal border. Echocardiography confirms tetralogy of Fallot. The structure marked **A** in the diagram is a large subaortic ventricular septal defect. Which of the following best explains why this defect, combined with right ventricular outflow tract obstruction, results in the cyanosis observed in this patient?
A. Left-to-right shunting of oxygenated blood across the VSD leading to pulmonary overcirculation and systemic desaturation
B. Right-to-left shunting of deoxygenated blood across the VSD into the systemic circulation due to elevated RV pressure from RVOT obstruction
C. Mixing of systemic and pulmonary venous return at the level of the right atrium due to an atrial septal defect
D. Obstruction of the pulmonary valve preventing adequate pulmonary blood flow and causing systemic venous congestion
Explanation
Why "Right-to-left shunting of deoxygenated blood across the VSD into the systemic circulation due to elevated RV pressure from RVOT obstruction" is right
The large nonrestrictive subaortic VSD (marked A) is one of the four cardinal features of tetralogy of Fallot. In isolation, a large VSD would produce left-to-right shunting. However, the presence of severe right ventricular outflow tract obstruction (infundibular pulmonary stenosis) raises RV systolic pressure above left ventricular pressure, reversing the shunt direction. Deoxygenated blood from the right ventricle is forced across the VSD into the left ventricle and aorta, bypassing the lungs entirely. This right-to-left shunt is the pathophysiologic basis of cyanosis in tetralogy of Fallot. The degree of RVOT obstruction determines severity of shunting and cyanosis (Nelson Pediatrics 22e; AHA/ACC Adult Congenital 2018).
Why each distractor is wrong
Left-to-right shunting of oxygenated blood across the VSD leading to pulmonary overcirculation and systemic desaturation: This describes an acyanotic VSD without significant RVOT obstruction. In tetralogy of Fallot, the RVOT obstruction is severe enough to reverse the shunt direction, producing right-to-left flow and cyanosis, not left-to-right flow.
Obstruction of the pulmonary valve preventing adequate pulmonary blood flow and causing systemic venous congestion: While pulmonary stenosis is present in tetralogy of Fallot, it is infundibular stenosis (narrowing of the RV outflow tract muscle), not primary pulmonary valve obstruction. The mechanism of cyanosis is shunt reversal across the VSD, not simple obstruction to pulmonary flow.
Mixing of systemic and pulmonary venous return at the level of the right atrium due to an atrial septal defect: Tetralogy of Fallot does not include an atrial septal defect as part of its four cardinal features. The mixing occurs at the ventricular level across the VSD, not at the atrial level.
High-YieldNEET PG
In tetralogy of Fallot, cyanosis results from RIGHT-TO-LEFT shunting across the VSD driven by elevated RV pressure from RVOT obstruction—the degree of stenosis determines the degree of cyanosis.
Nelson Pediatrics 22e; AHA/ACC Adult Congenital 2018
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