<prompt> Which of the following is the main factor for the ductal closure postnatallyIncrease in paial pressure of oxygen PaO2Increase in systemic vascular resistanceIncrease in circulating prostaglandin levelsDecrease in pulmonary venous resistance  <response>: 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 