**Core Concept**
The closure of the ductus arteriosus is a vital physiological process that occurs in the fetal and early neonatal periods. It is a result of the transition from a high-resistance, high-pressure fetal circulation to a low-resistance, low-pressure postnatal circulation. The ductus arteriosus is a vital shunt that connects the pulmonary artery to the aortic arch in the fetus, allowing blood to bypass the lungs and flow directly to the systemic circulation.

**Why the Correct Answer is Right**
The closure of the ductus arteriosus is primarily stimulated by the increase in oxygen levels in the blood after birth. This increase in oxygen tension activates the enzyme phosphodiesterase-5 (PDE-5), which breaks down cyclic guanosine monophosphate (cGMP) and leads to a decrease in the production of nitric oxide (NO). The decrease in NO production causes the smooth muscle cells of the ductus arteriosus to contract, leading to its closure.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased pressure in the pulmonary artery is not a direct stimulus for closure of the ductus arteriosus. While increased pressure in the pulmonary artery may contribute to the closure of the ductus arteriosus, it is not the primary stimulus.

**Option B:** Decreased oxygen levels in the blood would actually prevent the closure of the ductus arteriosus, as the ductus arteriosus remains patent in conditions of low oxygen levels, such as in congenital heart disease.

**Option C:** Increased prostaglandin E2 (PGE2) levels would actually prevent the closure of the ductus arteriosus, as PGE2 causes vasodilation and maintains the patency of the ductus arteriosus.

**Clinical Pearl / High-Yield Fact**
The closure of the ductus arteriosus is a critical physiological process that is essential for the transition from a fetal to a neonatal circulation. Failure of the ductus arteriosus to close can lead to patent ductus arteriosus (PDA), a congenital heart defect that can cause significant morbidity and mortality in infants.

**Correct Answer:** D. Increase in oxygen levels in the blood.