Which of the following is a common sign of dehydration?

Dry mucous membranes serve as a common sign of dehydration because they reflect a deficiency in body fluids. When the body is dehydrated, it does not have enough water to keep all tissues properly hydrated, leading to a noticeable dryness in the mucous membranes, such as those found in the mouth and nasal passages. The body's effort to conserve water during dehydration results in reduced saliva production, contributing to this dryness.

Increased urination typically occurs when there is excess fluid intake or when diuretics are involved, rather than from dehydration. Edema is swelling that usually signifies fluid retention, often in conditions where there is an imbalance or excess of fluid rather than a deficiency. Weight gain could indicate fluid retention or an increase in body mass but does not align with the body’s response to dehydration, which typically results in weight loss due to fluid loss.