Which of the following is a sign of dehydration that can lead to a heat-related illness?

Bright yellow urine is an indicator of dehydration because it signifies that the body is concentrated in its waste due to insufficient fluid intake. When hydration levels drop, the kidneys conserve water, leading to a higher concentration of waste products in urine, resulting in a darker, more vivid yellow color. This change is often one of the early warning signs of dehydration, which can increase the risk of heat-related illnesses such as heat exhaustion and heat stroke, particularly during exercise or in hot weather conditions.

In contrast, the other options do not typically indicate dehydration. An increased appetite may occur for a variety of reasons and does not reflect an individual's hydration status. Frequent urination typically indicates proper hydration, as the body is processing and expelling excess fluids. Wet skin is generally a sign of adequate hydration, especially during sweating, which is the body’s mechanism to cool itself and regulate temperature. Understanding these signs is crucial to prevent severe health complications during physical activity.