Which of the following is a symptom of dehydration?

Loss of appetite is indeed a symptom of dehydration. When the body lacks adequate hydration, it may not function properly, which can affect various physiological processes, including hunger signals. Dehydration can lead to a decrease in appetite as the body prioritizes essential functions needed for survival over the desire or need to eat. Additionally, when dehydrated, the body may experience less efficient digestion or metabolic processes, further contributing to the decrease in appetite.

The other symptoms listed, while they can be associated with various health conditions, do not specifically signify dehydration as clearly as loss of appetite does. Rapid heart rate can occur for several reasons, including anxiety or exertion, while increased energy is generally not associated with dehydration, as lack of fluids can lead to fatigue and lethargy. Excessive sweating might indicate excessive heat or exertion rather than dehydration specifically, as the body sweats to cool itself down.