At high altitudes, mountaineers and travellers may suffer from altitude sickness. This condition is mainly caused by which of the following?

Introduction / Context:
Altitude sickness, also called acute mountain sickness, is a common problem for people who travel rapidly to high mountains without proper acclimatisation. Symptoms include headache, nausea, dizziness, and fatigue. This question tests your understanding of the basic physiological cause of altitude sickness, which involves changes in the partial pressure of gases in the atmosphere, particularly oxygen.

Given Data / Assumptions:

• The context is high altitudes, such as mountains above a few thousand metres.
• Options mention high or low partial pressures of oxygen and carbon dioxide, and low haemoglobin levels.
• We assume a person who has ascended relatively quickly from low altitude to high altitude, without long term adaptation.

Concept / Approach:
Atmospheric pressure decreases with altitude. While the percentage of oxygen in the air remains approximately constant at about 21 percent, the total atmospheric pressure is lower at high altitude, so the partial pressure of oxygen (the portion of total pressure contributed by oxygen) is also lower. This reduced partial pressure of oxygen means that less oxygen diffuses from the lungs into the blood with each breath, leading to tissue hypoxia. The body attempts to compensate by increasing breathing and heart rate, but if ascent is too rapid, symptoms of altitude sickness develop. Haemoglobin levels are not initially low; in fact, over time, the body produces more red blood cells to adapt. The partial pressure of carbon dioxide is usually lower at high altitude because increased breathing (hyperventilation) blows off more carbon dioxide. Therefore, the main cause of altitude sickness is low partial pressure of oxygen, not high oxygen, low haemoglobin, or high carbon dioxide.

Step-by-Step Solution:

Verification / Alternative check:
Physiology textbooks link acute mountain sickness to hypobaric hypoxia, which means low oxygen pressure due to reduced barometric pressure. They explain that acclimatisation involves increased ventilation, increased red blood cell production, and other adaptations to improve oxygen delivery. Medical advice for high altitude travel emphasises gradual ascent and sometimes prophylactic medications to reduce the risk of altitude sickness, all of which are measures to cope with low oxygen availability, not with high oxygen or carbon dioxide. This evidence confirms that low partial pressure of oxygen is the main factor responsible for altitude sickness.

Why Other Options Are Wrong:

High partial pressure of oxygen at high altitude is wrong because atmospheric and oxygen partial pressures both decrease with altitude, not increase.

Low level of haemoglobin in all high altitude residents is incorrect; people who live at high altitude often have higher haemoglobin levels as an adaptation, and visitors do not suddenly lose haemoglobin when they ascend.

High partial pressure of carbon dioxide at high altitude is also wrong because hyperventilation at altitude tends to lower carbon dioxide levels, not raise them.

Common Pitfalls:
Students sometimes confuse oxygen concentration with partial pressure and may think that because oxygen is always about 21 percent of the air, it should not cause problems. The key is that the partial pressure (absolute pressure) of oxygen matters for gas exchange, not just its percentage. Another pitfall is to mix up altitude sickness with anaemia, which also reduces oxygen carrying capacity but has a different cause. To avoid these errors, remember that altitude sickness is fundamentally due to low partial pressure of oxygen at high altitudes, causing inadequate oxygenation of body tissues.

Final Answer:
Therefore, altitude sickness at high altitude is mainly caused by low partial pressure of oxygen.