Which of the following is a sign of dehydration?

Decreased urine output is indeed a sign of dehydration. When the body is dehydrated, it conserves water to maintain essential physiological functions, leading to less urine production. This occurs because the kidneys respond to low fluid levels by reabsorbing more water and reducing the volume of urine that is excreted.

In a well-hydrated state, the body typically produces a sufficient amount of urine to eliminate waste and maintain fluid balance. However, when fluid intake is inadequate due to factors like excessive heat, exercise, or illness, the body's compensatory mechanisms kick in, leading to a significant decrease in urine output. This condition serves as an important indicator of hydration status, highlighting that the body is prioritizing the retention of fluids.