HYPOXIA
Hypoxia is a medical condition where there is a deficiency of oxygen in the tissues and organs of the body. This can occur due to various reasons, including respiratory disorders, circulatory problems, or exposure to high altitudes. Hypoxic hypoxia is caused due to the reduction of oxygen entering the blood due to low partial pressure of oxygen with altitude. It is caused when flying a depressurized aircraft at high altitudes and can lead to symptoms such as shortness of breath, rapid heartbeat, confusion, headache, dizziness, impaired judgement, loss of consciousness and eventually death.

Dealing with the hypoxia symptoms early is critical since the Time of Useful Consciousness (the duration during which an individual can effectively carry out flying duties in conditions where oxygen levels are insufficient) is usually very short. The steps to be followed in case of hypoxia symptoms are:

1. Don oxygen mask and breath 100% oxygen. This ensures that the hypoxia symptoms will be eliminated.
2. Commence a descent below 10 000 ft and land as soon as possible. Descending to a lower altitude increases atmospheric pressure, which restores the oxygen pressure as well and hypoxia symptoms will not reappear even without the oxygen mask.
3. Seek medical advice after landing. Seeing a doctor is important to make sure hypoxia is treated.

The question is asking about the MOST DANGEROUS reaction by the pilot. Let's analyze the given options:

Descend to land as soon as possible. → INCORRECT. Descending to a lower altitude increases atmospheric pressure, which restores the oxygen pressure as well and will help hypoxia symptoms to become eliminated. This is not the most dangerous reaction.

Don oxygen mask and descend. → INCORRECT. This is the right course of action which would not lead to any dangerous outcome.

Divert immediately to the nearest aerodrome. → INCORRECT. Diverting to the nearest aerodrome means that the pilot will descent and land. Descending to a lower altitude increases atmospheric pressure, which restores the oxygen pressure as well and will help hypoxia symptoms to become eliminated. This is not the most dangerous reaction.

Maintain altitude and control of the aircraft. → CORRECT. This is not the right reaction to the hypoxia symptoms. Dealing with the hypoxia symptoms early is critical since the Time of Useful Consciousness is usually very short. The pilot should don the oxygen mask and descent, otherwise the hypoxia symptoms will aggravate and loss of consciousness will occur.