**Core Concept**
The ductus arteriosus is a fetal blood vessel that connects the pulmonary artery to the aortic arch, allowing blood to bypass the lungs. After birth, it normally closes due to increased oxygenation. The underlying principle being tested involves the factors that influence the patency of the ductus arteriosus.

**Why the Correct Answer is Right**
Prostaglandin E2 (PGE2) is known to keep the ductus arteriosus open. However, the most relevant factor in maintaining its patency, especially in the context of a question looking for a singular answer, involves prostaglandins, particularly PGE2, which is produced by the placenta and the ductus arteriosus itself. This mechanism is crucial for fetal circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while oxygen levels influence the closure of the ductus arteriosus, they are not directly responsible for keeping it open.
**Option B:** Incorrect as it's not directly related to maintaining the ductus arteriosus open.
**Option C:** Incorrect because it's the decrease in prostaglandins after birth, due to the loss of the placental source and increased oxygenation, that leads to the closure of the ductus arteriosus.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that non-steroidal anti-inflammatory drugs (NSAIDs) like indomethacin can promote the closure of a patent ductus arteriosus by inhibiting prostaglandin synthesis.

**Correct Answer:** D. Prostaglandin.