A nurse is obtaining a 4-month-old infant's vital signs. The infant's heart rate is 190/min, and her temperature is 40° C (104° F). The father asks the nurse, "Why is my baby's heart beating so fast?" Which of the following is an appropriate response by the nurse?

A. "As your baby begins to fall asleep, her heart rate will decrease. This statement may be true in a general sense, but it does not address the parent's specific concern about the cause of tachycardia in the context of fever. It's vague and dismissive of the elevated temperature. B. "Your baby's heart is beating fast in an attempt to cool down the body." While increased heart rate can be part of the body's response to fever, the primary reason is not to "cool down" the body but to increase metabolism and circulation. This explanation is not entirely accurate. C. "This is within the expected range for your baby." A heart rate of 190/min at rest is above the normal range for a 4-month-old, especially in the context of fever. Normal resting heart rate for this age is approximately 100-160 bpm. This response is inaccurate and misleading. D. "The fever is causing an increase in your baby's heart rate." Fever increases metabolic demand, which leads to an elevated heart rate (tachycardia). This is a clear, accurate, and appropriate explanation for the parent.