Which of the following is a common sign of dehydration?

Thirst is a well-known and primary indicator of dehydration. When the body experiences a shortage of fluids, it activates biological signals to prompt the individual to drink more water and restore hydration levels. This sensation is driven by the hypothalamus, which detects changes in blood osmolarity and blood volume. Essentially, when dehydration begins to set in, the body's need for water becomes evident through increased thirst.

In contrast, the other options do not align with typical signs of dehydration. Increased appetite can occur for various reasons unrelated to hydration status; bright urine generally indicates adequate hydration due to the concentration of waste products being lower in well-hydrated individuals; and high energy levels are typically associated with proper hydration and nutrition rather than a sign of dehydration. Thus, your selection of thirst as the sign of dehydration is well-founded.