Ductus aeriosus closes in response to ?
**Core Concept**
The ductus arteriosus is a vital fetal blood vessel that connects the pulmonary artery to the aorta, allowing blood to bypass the lungs and oxygenate in the placenta. After birth, the ductus arteriosus must close to prevent excessive blood flow to the lungs and ensure proper circulation.
**Why the Correct Answer is Right**
The ductus arteriosus closes in response to an increase in oxygen tension in the blood, which stimulates the production of prostaglandin synthase and subsequent conversion to prostaglandin E2 (PGE2). PGE2 causes vasodilation of the ductus arteriosus, but when oxygen levels rise, PGE2 production decreases, and the ductus arteriosus constricts. This process is mediated by the enzyme cyclooxygenase (COX), which is inhibited by nonsteroidal anti-inflammatory drugs (NSAIDs), thereby delaying ductal closure.
**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as this is a pharmacological effect rather than a physiological response.
* **Option B:** While vasopressin does cause vasoconstriction, it is not directly involved in ductus arteriosus closure.
* **Option C:** Although prostaglandin E2 is involved in ductus arteriosus closure, it is not the primary stimulus for closure.
**Clinical Pearl / High-Yield Fact**
The ductus arteriosus typically closes within 10-15 hours of birth, but delayed closure can be caused by conditions such as respiratory distress syndrome, congenital heart disease, or maternal exposure to NSAIDs.
**Correct Answer: D. Increase in oxygen tension.**