A nurse is obtaining vital signs from the 2-month-old infant. The infant's heart rate is 190/min, and his temperature is 40° C (104° F). The father asks the nurse why the infant's heart is beating so fast. Which of the following responses by the nurse is appropriate?

A. "The fever is causing an increase in your baby's heart rate." Fever increases metabolic demand, which commonly leads to tachycardia in infants as the body attempts to regulate temperature and support circulation. This explanation directly addresses the physiological cause and is appropriate. B. "This is within the expected range for your baby." A heart rate of 190/min in a 2-month-old is elevated, especially in the presence of a fever. While infants have higher baseline heart rates (120-160 b/min), this value is elevated and requires clinical interpretation, not reassurance. C. "Your baby's heart is beating fast in an attempt to cool down his body.": While increased cardiac output may help distribute heat, the primary cause of tachycardia is metabolic demand from fever—not a direct mechanism to cool the body. D. "As your baby begins to fall asleep, his heart rate will decrease." Although heart rate may decrease with sleep, this does not address the father's concern about the elevated rate now, and it downplays the need to monitor the infant in the context of high fever