**Core Concept**
At high altitude, the partial pressure of oxygen in arterial blood (PaO2) decreases due to the lower atmospheric pressure and lower oxygen concentration. This physiological response is a result of the body's attempt to adapt to the hypoxic environment.

**Why the Correct Answer is Right**
The decrease in PaO2 at high altitude occurs due to the lower atmospheric pressure, which reduces the oxygen saturation in the lungs. This leads to a decrease in the diffusion of oxygen from the lungs into the bloodstream, resulting in lower PaO2 levels. The body attempts to compensate for this decrease by increasing red blood cell production and cardiac output to deliver more oxygen to tissues.

**Why Each Wrong Option is Incorrect**
**Option B:** High PaO2 is incorrect because high altitude is characterized by low oxygen levels, not high.
**Option C:** Normal PaO2 is incorrect because the decrease in atmospheric pressure at high altitude leads to a decrease in PaO2.
**Option D:** High PaCO2, Low PaO2 is incorrect because while PaO2 does decrease at high altitude, PaCO2 typically increases due to hypoventilation, not high.

**Clinical Pearl / High-Yield Fact**
It's essential to note that acclimatization to high altitude involves a complex physiological response, including increased red blood cell production, increased cardiac output, and changes in respiratory rate and depth. Understanding these adaptations is crucial for managing patients with altitude-related illnesses.

**✓ Correct Answer: A. Low PaO2**