**Core Concept**
The question is testing the understanding of the effects of altitude on the body's physiological adaptations. At high altitudes, the lower oxygen levels lead to increased red blood cell production to compensate for the reduced oxygen availability. Conversely, living at sea level allows for normal oxygen levels, resulting in no such compensatory mechanisms.

**Why the Correct Answer is Right**
The correct answer is related to the body's adaptation to low oxygen levels at high altitudes. At sea level, the man's body does not need to produce more red blood cells to compensate for low oxygen levels. In contrast, his twin brother living at high altitude has increased erythropoietin production, leading to an increase in red blood cell mass. This results in a higher hematocrit (the proportion of blood volume made up by red blood cells) in the twin brother. Hematocrit is a reflection of the body's ability to adapt to low oxygen levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemoglobin concentration can increase at high altitudes due to increased red blood cell production, but it is not a direct reflection of the body's adaptation to low oxygen levels. Hematocrit is a more accurate indicator.
**Option B:** Oxygen saturation is a measure of the amount of oxygen bound to hemoglobin in the blood. It would be higher in the twin brother living at high altitude, not lower.
**Option C:** Arterial oxygen tension (PaO2) would be lower in the twin brother living at high altitude, not higher.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the body's adaptation to high altitude takes time. Acclimatization to high altitude can take several weeks, and the body's compensatory mechanisms are not fully developed in the first few days.

**Correct Answer:** C. Hematocrit.