**Core Concept**
The ductus arteriosus is a vital fetal blood vessel connecting the pulmonary artery to the aortic arch, allowing blood to bypass the lungs. After birth, it typically closes within the first few days of life. The mechanism of closure involves a decrease in prostaglandin E2 (PGE2) levels, which relax and dilate the ductus arteriosus, and an increase in oxygen levels, which stimulate the ductus arteriosus to close.

**Why the Correct Answer is Right**
Indomethacin is a nonsteroidal anti-inflammatory drug (NSAID) that inhibits cyclooxygenase (COX) enzymes, which are responsible for converting arachidonic acid into prostaglandins. By reducing PGE2 production, indomethacin therapy promotes ductus arteriosus closure. This effect is particularly useful in neonates with ductal-dependent congenital heart defects, where closure of the ductus arteriosus is critical for successful surgical management.

**Why Each Wrong Option is Incorrect**
**Option A:** Decrease in peripheral oxygen saturation would actually promote ductus arteriosus patency, as the ductus arteriosus remains open in response to low oxygen levels.
**Option C:** Prostaglandin E1 (PGE1) is actually used to maintain ductus arteriosus patency in neonates with ductal-dependent congenital heart defects, rather than promoting closure.
**Option D:** Increase in pulmonary vascular resistance would not directly cause ductus arteriosus closure; instead, it would lead to increased right ventricular afterload.

**Clinical Pearl / High-Yield Fact**
Indomethacin therapy is a key treatment for promoting ductus arteriosus closure in neonates with ductal-dependent congenital heart defects. However, it is essential to weigh the benefits against potential risks, such as renal impairment and oligohydramnios in the fetus.

✓ Correct Answer: B. Indomethacin therapy