A nurse is monitoring a 6-month-old infant 20 min after administering a hepatitis B immunization. Which of the following findings is the nurse's priority?

A. Temperature 37.7° C (99.9° F): A mild temperature elevation is a common, non-urgent reaction to vaccination. It does not indicate a serious adverse effect and does not require immediate intervention. B. Hives on the child's neck: Hives can be an early sign of an anaphylactic reaction, which is life-threatening and requires immediate intervention. This is the priority finding and warrants prompt evaluation and treatment. C. Prolonged crying: While excessive crying may indicate discomfort or pain, it is not an immediate threat to the infant’s airway, breathing, or circulation. It should be monitored but is not the top concern. D. Redness at the injection site: Local redness is a typical and mild side effect of many vaccines, including hepatitis B. It usually resolves without intervention and is not considered an emergency.