This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Pediatrics
Which of the following is the main factor for the ductal closure postnatally 
Select and explain. 
A. Increase in paial pressure of oxygen (PaO2), B. Increase in systemic vascular resistance, C. Increase in circulating prostaglandin levels, D. Decrease in pulmonary venous resistance
A: Increase in paial pressure of oxygen (PaO2)
Answer: A: Increase in paial pressure of oxygen (PaO2) 
Explanation: Oxygen is the most impoant controlling ductal closure. Note: Gestational age also appears to play an impoant role; the ductus of a premature infant is less responsive to oxygen, even though its muscular is developed. Hence, patent ductus aeriosus (PDA) is more common in premature infants. Ref: Nelson textbook of pediatrics 21st edition Pgno: 2345