What is the main cause of hypoxia?

The main cause of hypoxia is inadequate oxygen supply to the tissues. Hypoxia occurs when there is a deficiency of oxygen in the tissues, leading to impaired cellular function and metabolic processes. This can happen for various reasons, such as respiratory issues that limit oxygen uptake, circulatory problems that reduce blood flow, or environmental factors like high altitudes where oxygen availability is lower.

In this context, other options do not directly define or cause hypoxia. Excessive physical activity can lead to temporary increases in oxygen demand but is not a direct cause of tissue hypoxia unless coupled with an inability to intake or utilize oxygen effectively. High carbon dioxide levels in the blood indicate issues with gas exchange but are a result rather than a primary cause of hypoxia. Allergies to certain medications could lead to respiratory distress or anaphylactic reactions, potentially causing hypoxia in extreme cases, but they are not a common or direct cause of tissue oxygen deficiency. Hence, inadequate oxygen supply is the primary factor that results in hypoxia.