'Altitude sickness' is caused at high altitude due to

The correct answer is B, low partial pressure of oxygen. As one climbs, air pressure falls, so each breath delivers oxygen at a lower partial pressure and less of it passes from the lungs into the blood. The share of oxygen in air stays about 21 percent at all heights, but at around 5,500 metres the total pressure is roughly half that at sea level, and so is the oxygen pressure. The resulting shortage of oxygen in the tissues, called hypoxia, causes headache, nausea, breathlessness, dizziness and poor sleep, and in severe cases fluid in the lungs or brain. The body adapts over days by breathing faster and making more red blood cells, which is why climbers acclimatise in stages. Option A is wrong because oxygen pressure falls, not rises, at height. Option C is wrong because haemoglobin is not the cause; in fact it rises during acclimatisation, and the problem is the thin air. Option D is wrong because carbon dioxide pressure also falls at altitude, as faster breathing washes it out. Exam tip: altitude sickness = hypoxia from low partial pressure of O2; the body responds with more red blood cells (erythropoietin).