Fetal Circulation and Transitional Changes at Birth

A 28-week-old fetus is being evaluated on prenatal ultrasound. The sonographer notes that the foramen ovale is patent and functionally open, allowing right-to-left shunting of blood. Which of the following best explains the physiological significance of this shunt in fetal circulation?

A 2-day-old male neonate born at term to a 28-year-old primigravida presents with persistent cyanosis (SpO₂ 78% on room air) and mild respiratory distress. On examination, the infant has a single loud S₂ and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows an 'egg-on-string' appearance with pulmonary plethora. The ductus venosus has already closed. Which fetal vascular structure is most likely still patent and responsible for the right-to-left shunt in this neonate?

A 6-hour-old term female neonate born to a 32-year-old mother develops sudden onset of severe respiratory distress and shock. On examination, the infant is pale, hypotensive (BP 45/28 mmHg), with weak pulses and poor perfusion. Auscultation reveals muffled heart sounds. Chest X-ray shows cardiomegaly with pulmonary edema. Echocardiography reveals a large left-to-right shunt at the ductal level with reversal of flow in the descending aorta. Which of the following best explains the acute hemodynamic collapse in this neonate?

Regarding the fetal circulation and the changes that occur at birth, all of the following statements are correct EXCEPT:

A newborn infant is examined 2 hours after vaginal delivery. Auscultation reveals a continuous 'machinery' murmur at the left infraclavicular area. Which of the following is the most common site of patency in persistent fetal circulation?