Which of the following is a common sign of dehydration during exercise?

Dark yellow urine is a common sign of dehydration during exercise because it indicates a higher concentration of waste products in the urine due to insufficient fluid intake. When the body is adequately hydrated, urine typically appears light yellow or clear, reflecting a proper balance of water and nutrients in the body. Conversely, during exercise, as sweat is lost and fluids are not replenished, the kidneys conserve water, resulting in darker urine. Monitoring urine color is a practical and easy way to assess hydration status, making this an important indicator for individuals engaged in physical activities.

Other options do not serve as direct indicators of dehydration. Elevated heart rate can occur for various reasons during exercise, not exclusively due to dehydration. Increased appetite might actually occur in well-hydrated individuals or after exercise, as the body seeks to replenish energy stores, while swelling of muscles could suggest overhydration or an injury rather than dehydration.