Which of the following is a sign of severe allergic reaction that requires immediate attention?

Swelling of the lips is a significant sign of a severe allergic reaction, often indicative of anaphylaxis, a potentially life-threatening condition. This reaction can lead to swelling in the throat and airway, which can obstruct breathing. It represents a serious reaction that can escalate quickly, making immediate medical attention crucial to ensure the safety and well-being of the individual experiencing these symptoms. Recognizing this sign allows for swift intervention, such as administering epinephrine, which is vital in managing severe allergic reactions effectively.

In contrast, itchy skin, nausea, and rash, while they may indicate an allergic response, are generally less severe on their own and do not typically require the same level of urgency as swelling of the lips. These symptoms can still be uncomfortable and may warrant attention, but they do not suggest the imminent danger posed by airway obstruction associated with severe allergic reactions.