**Core Concept**
At high altitudes, the body adapts to decreased oxygen availability, leading to changes in respiratory and metabolic processes.

**Why the Correct Answer is Right**
As the mountaineer ascends to 18,000 feet, the barometric pressure decreases, resulting in a reduction in inspired oxygen (FiO2). This decrease in FiO2 leads to a decrease in arterial oxygen tension (PaO2). The body compensates for this decrease by increasing ventilation, which in turn increases PaCO2. However, the increase in PaCO2 is minimal compared to the decrease in PaO2. The pH remains relatively stable due to the bicarbonate buffering system.

**Why Each Wrong Option is Incorrect**
**Option A:** ↑ed PaCO2 - While PaCO2 does increase, it is not the most significant change at high altitude.
**Option B:** ↓ed Barometric pressure - This is a correct statement, but not the most relevant change in this context.
**Option C:** ↓ed Inspired O2 - This is a contributing factor, but not the direct answer.
**Option D:** ↓ed PaO2 - This is the most direct and significant change at high altitude.

**Clinical Pearl / High-Yield Fact**
At high altitudes, the body's ability to adapt to hypoxia is limited by the rate of ascent and individual variability in physiological response.

**Correct Answer:** D. ↓ed PaO2