Which physiologic change triggers the closure of the ductus arteriosus after birth?

• A. Increased oxygen tension with decreased prostaglandins due to cord clamping
• B. Decreased oxygen tension
• C. Increased prostaglandins
• D. No change in prostaglandins

Answer: (D)

After birth the ductus arteriosus closes mainly because the lungs take over oxygenation and placental support for prostaglandins ends. With the cord clamped, pulmonary blood flow increases and arterial oxygen tension rises. At the same time, the source of placental prostaglandins is removed, so circulating prostaglandin levels fall. The higher oxygen tension directly stimulates smooth muscle constriction of the ductus arteriosus, and the drop in prostaglandins removes the hormone-driven relaxation that kept it open. This combination—more oxygen and less prostaglandins—triggers functional closure.

The other scenarios don’t fit: lower oxygen tension would favor keeping it open; higher prostaglandins would continue to keep it patent; and no change in prostaglandins wouldn’t produce the needed trigger.