Which finding indicates that a 6-month-old infant has moderate dehydration?

The finding that indicates a 6-month-old infant has moderate dehydration is the presence of dry mucous membranes. When an infant is moderately dehydrated, one of the key signs is the condition of their mucous membranes. Inadequate fluid intake often leads these membranes to become dry and less moist, which is a clear indicator of dehydration.

While tachycardia, tachypnea, and decreased urine output are also signs that may suggest dehydration, they are more general and can occur due to a variety of other reasons related to the infant's overall health status. For example, an increased heart rate (tachycardia) can occur due to fever, anxiety, or other physiological factors, and does not specifically point to moderate dehydration. Similarly, rapid breathing (tachypnea) can be attributed to respiratory issues or other medical conditions beyond dehydration. Decreased urine output is a critical symptom of dehydration, but it can also indicate other issues with renal function or fluid balance.

In contrast, the specific observation of dry mucous membranes provides a direct indication of the body’s fluid status and is a commonly recognized clinical sign used to gauge dehydration levels in infants. Therefore, this choice is the most indicative of moderate dehydration in a 6-month-old.