**Core Concept**
Hypoxic hypoxia, also known as hypoxemic hypoxia, occurs when there is an inadequate supply of oxygen to the body's tissues. This can be due to various factors such as high altitude, anemia, or respiratory diseases. The underlying principle being tested is the ability to identify the most reliable indicator of hypoxic hypoxia.

**Why the Correct Answer is Right**
The best parameter for analyzing hypoxic hypoxia is PaO2, as it directly measures the partial pressure of oxygen in arterial blood. This parameter is crucial in assessing the severity of hypoxia and guiding therapeutic decisions. PaO2 is a direct reflection of the oxygen available to the body's tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it may not directly reflect oxygen levels.
**Option B:** Incorrect as it is not a direct measure of oxygen in the blood.
**Option C:** Incorrect because, although related, it is not the most direct measure of hypoxic hypoxia.

**Clinical Pearl / High-Yield Fact**
In clinical practice, a PaO2 level below 80 mmHg is generally considered indicative of hypoxia. This parameter is critical in managing patients with respiratory or cardiac conditions.

**Correct Answer:** D. PaO2.