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 6-hour-old female neonate born vaginally at 38 weeks in Mumbai is noted to have mild cyanosis that improves with crying and oxygen supplementation. Physical examination reveals a continuous 'machinery' murmur best heard at the left infraclavicular region. The infant's pulses are bounding, and there is a wide pulse pressure (systolic 95 mmHg, diastolic 40 mmHg). Chest X-ray shows a 'boot-shaped' heart with pulmonary plethora. Which of the following best explains the bounding pulses and wide pulse pressure observed in this neonate?