Which of the following is a key sign of dehydration in a patient?

Dry mucous membranes are a key sign of dehydration because they indicate a lack of adequate fluid in the body. When a person is dehydrated, the body's fluid levels drop, leading to reduced moisture in tissues, including those in the mucous membranes of the mouth and throat. This is often assessed by visual inspection during a physical examination, and the presence of dryness can be a clear indicator that the patient may not be adequately hydrated.

In contrast, excessive sweating typically suggests that the body is attempting to regulate temperature and can occur even in well-hydrated individuals, depending on the environmental conditions. Increased urine output is generally a sign of fluid overload or effective kidney function and is not indicative of dehydration. Clear urine suggests adequate hydration and the clearance of wastes, making it the opposite of what you would expect in a dehydrated state. Hence, the identification of dry mucous membranes serves as an important clinical sign for assessing hydration status in patients.