Which symptom indicates heat exhaustion?

Heat exhaustion occurs as a result of prolonged exposure to high temperatures, often in combination with high humidity and physical exertion. It is characterized by several symptoms that indicate the body's ability to regulate temperature is overwhelmed.

The symptom of tachycardia (increased heart rate) and weakness is indicative of heat exhaustion because the body attempts to cool itself through increased blood flow to the skin, which can lead to cardiovascular strain. As blood vessels dilate to release heat, the heart beats faster to maintain adequate blood circulation. Additionally, weakness signifies that the body is beginning to lose its ability to cope with the heat, as dehydration and electrolyte imbalances set in.

Other symptoms of heat exhaustion may include excessive sweating, dizziness, headache, nausea, and muscle cramps. However, in this context, the combination of tachycardia and weakness specifically highlights the cardiovascular effects that manifest when the body is under significant stress from heat exposure, making it the best indicator of heat exhaustion among the choices presented.