What is a key sign of dehydration in infants?

A sunken fontanelle is a key sign of dehydration in infants because this condition can lead to fluid loss that affects the overall hydration balance in the body. The fontanelles are the soft spots on an infant's head where the skull bones have not yet fused together. When an infant is dehydrated, the body loses more fluid than it takes in, leading to a decrease in the cerebrospinal fluid that helps to keep the brain cushioned. This results in a noticeable depression in the fontanelle area, indicating that the infant is not retaining enough fluid.

In contrast, other signs such as weight gain, frequent soiled diapers, or excessive tear production reflect different health conditions. Weight gain typically indicates good nutrition and hydration status, while frequent soiled diapers might suggest adequate fluid intake. Excessive tear production often suggests a well-hydrated state, as tears are primarily composed of water. Therefore, the presence of a sunken fontanelle is a crucial and clinical indicator used to assess dehydration in infants, making it a vital sign for healthcare professionals to watch for in pediatric emergency situations.