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Heat exhaustion and heat stroke are primarily caused by a lack of fluid intake, particularly in conditions of high temperature and humidity. When the body becomes dehydrated, it loses the ability to cool itself effectively through sweating. This leads to an increase in body temperature and places additional stress on the cardiovascular system, which can result in heat exhaustion. If the body's temperature regulation fails, it can progress to heat stroke, a more severe condition that can be life-threatening.

While excessive exercise and poor ventilation can contribute to these conditions, they are not the main causes. Cold temperatures would not typically lead to heat exhaustion or heat stroke, as these conditions arise from excessive heat and inadequate hydration. Therefore, the most critical factor among the options listed is indeed a lack of fluid intake, which directly impacts the body's ability to dissipate heat and maintain a safe core temperature.