Which of the following is a sign of an allergic reaction?

The presence of hives, swelling, and difficulty breathing are key indicators of an allergic reaction. Hives, which are raised, red welts on the skin, often appear as a response to allergens, and swelling can occur in various parts of the body, especially around the eyes, lips, or throat. Difficulty breathing is a critical sign that the airway may be affected, potentially indicating a more severe reaction, such as anaphylaxis. Identifying these symptoms is crucial for prompt and effective treatment, which may include the administration of antihistamines or epinephrine in severe cases.

In contrast, fever and fatigue, headache and dizziness, as well as nausea and vomiting, can be symptoms associated with various illnesses or conditions, but they are not specific indicators of allergic reactions. Hence, the correct answer highlights the distinct signs that are consistent with an allergic response, which are essential for first responders to recognize and act upon.