**Core Concept**
At high altitudes, the atmospheric pressure decreases, leading to a decrease in the partial pressure of oxygen (PO2) in the air we breathe, which in turn affects the amount of oxygen available for gas exchange in the lungs.

**Why the Correct Answer is Right**
The partial pressure of oxygen (PO2) in the air at sea level is approximately 159 mmHg. At an altitude of 6500 meters, the atmospheric pressure is 347 mmHg, which is roughly 23% of the sea-level pressure. To calculate the inspired PO2 at high altitude, we can use the formula: inspired PO2 = (atmospheric pressure at altitude / atmospheric pressure at sea level) x PO2 at sea level. Plugging in the numbers, we get inspired PO2 = (347 mmHg / 760 mmHg) x 159 mmHg ≈ 63 mmHg.

**Why Each Wrong Option is Incorrect**
**Option A:** This answer is too high because the atmospheric pressure at 6500 meters is significantly lower than at sea level, resulting in a lower inspired PO2.
**Option C:** This answer is too low because the calculation above shows that the inspired PO2 at 6500 meters is approximately 63 mmHg, not 53 mmHg.
**Option D:** This answer is too high because it does not take into account the decrease in atmospheric pressure at high altitude, which affects the partial pressure of oxygen in the air.

**Clinical Pearl / High-Yield Fact**
At high altitudes, the body adapts to the lower oxygen levels by increasing red blood cell production (polycythemia) and increasing cardiac output to compensate for the decreased oxygen delivery to tissues. This adaptation process can take several days to weeks to occur, and it is critical for climbers and individuals who work at high altitudes to be aware of these changes to prevent altitude sickness.

**✓ Correct Answer: B. 63 mm Hg**