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 CVS Embryology Q5  

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5. In the newborn nursery, a neonate develops a bluish color when crying. Which of the following congenital defects may be the cause of the cyanosis?

A. Patent ductus arteriosus

B. Persistent truncus arteriosus

C. Primum type atrial septal defect

D. Secundum type atrial septal defect

E. Ventricular septal defect




The correct answer is B. A persistent truncus arteriosus results from a failure of the
aorticopulmonary septum to form. The aorticopulmonary septum divides the truncus arteriosus into the ascending aorta and the pulmonary trunk. If this septum fails to form, the single truncus arteriosus will receive blood from both the right and left ventricles, allowing the deoxygenated blood to be mixed with the oxygenated blood. The systemic circulation therefore receives blood that is not fully
oxygenated, causing cyanosis.

A patent ductus arteriosus (choice A) results in blood passing from the aorta to the pulmonary trunk postnatally. This left-to-right shunt does not cause cyanosis. Cyanosis is caused by right-to-left shunts. Prenatally, the ductus arteriosus allows the passage of blood from the pulmonary trunk to the aorta. After birth, when the pressure gradient reverses and the pressure is higher on the left side of the heart, the flow in a patent ductus arteriosus reverses and becomes a left-to-right shunt.

Atrial septal defects (choices C and D) allow for left-to-right shunting of blood postnatally because the pressure is higher in the left atrium than in the right atrium. Only right-to-left shunts cause cyanosis.

Ventricular septal defects (choice E) also produce a left-to-right shunt because of the left-to-right pressure gradient. Therefore, these are noncyanotic defects.

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