## Core Concept
The question pertains to physiological changes that occur at or shortly after birth. These changes are crucial for the newborn's adaptation to extrauterine life and involve various systems, including the cardiovascular, respiratory, and renal systems.

## Why the Correct Answer is Right
The closure of the ductus arteriosus is a key event that occurs shortly after birth. At birth, with the first breaths, the lungs expand, and the resistance in the pulmonary vasculature drops. This decrease in pulmonary vascular resistance and the increase in oxygen tension in the blood lead to the functional closure of the ductus arteriosus, which typically becomes anatomically closed within a few weeks. This process is essential for the separation of pulmonary and systemic circulations.

## Why Each Wrong Option is Incorrect
- **Option A:** The increase in systemic vascular resistance also occurs at birth due to the removal of the low-resistance placental circulation from the systemic circuit. This change contributes to the closure of the ductus arteriosus.
- **Option B:** The decrease in pulmonary vascular resistance happens immediately after birth as the lungs expand and oxygenate the blood, leading to vasodilation of pulmonary vessels.
- **Option D:** The increase in PaO2 (partial pressure of oxygen in arterial blood) occurs with the initiation of breathing and expansion of the lungs, which is a critical trigger for ductus arteriosus closure.

## Clinical Pearl / High-Yield Fact
A clinically relevant point to remember is that the administration of indomethacin, a nonsteroidal anti-inflammatory drug (NSAID), can be used to pharmacologically close a patent ductus arteriosus (PDA) in preterm infants. The rationale is that prostaglandins, which are inhibited by NSAIDs, play a role in keeping the ductus arteriosus open.

## Correct Answer: D.