What is a critical sign of heart failure in infants?

Lethargy is a critical sign of heart failure in infants because it indicates a decrease in energy and responsiveness, which can be a result of inadequate blood flow and oxygenation to vital organs. In the context of heart failure, the heart's inability to pump effectively can lead to insufficient perfusion of tissues, causing the infant to be less active and more fatigued. This diminishment in energy levels is a direct response to the impaired cardiac output, and it often warrants immediate medical attention.

In contrast, other options do not represent typical signs of heart failure. Increased weight could occur due to fluid retention but is not a straightforward indicator of heart failure without additional context. An improved appetite unfortunately contradicts the general clinical signs of heart failure, as affected infants typically show a decrease in feeding and appetite due to fatigue or discomfort. Normal urine output can also be misleading; heart failure may lead to reduced renal perfusion, often resulting in decreased urine output rather than normal levels. Therefore, lethargy stands out as a significant and concerning indicator of heart failure that requires further evaluation and intervention.