What vital sign might indicate a fever?

A body temperature exceeding 100.4°F (38°C) is a clear indicator of a fever. Fever is typically recognized when the body's temperature rises above the normal range, which is generally around 98.6°F (37°C). This increase in temperature is often part of the body's natural response to infection or illness, as it can help to stimulate the immune system’s activity and create an environment less favorable for pathogens.

The other options do not directly indicate a fever. A decrease in respiratory rate can occur for various reasons, but it does not signify an elevated body temperature. Stable blood pressure readings are also unrelated to fever, as they reflect the pressure of the blood in the circulatory system rather than the body's temperature. An increased heart rate, while sometimes associated with fever, is not a definitive indicator on its own since it may result from numerous other conditions unrelated to temperature changes. Therefore, the increase in body temperature above the specified threshold is the clear and specific vital sign indicating a fever.