Which of the following is a symptom of a true allergic reaction?

A true allergic reaction is characterized by the body's immune system reacting to a substance (allergen) that it mistakenly identifies as harmful. Hives, also known as urticaria, are a classic symptom of an allergic reaction. They present as raised, itchy welts on the skin and occur due to the release of histamines and other chemicals from cells in response to an allergen. This immediate hypersensitivity reaction is a hallmark of allergies, distinguishing it from other types of symptoms that may arise from diverse medical conditions.

In contrast, gastrointestinal distress, fever, and headaches can be symptoms of various other conditions but are not specific to allergic reactions. Gastrointestinal distress may occur from food intolerances or infections, fever can indicate an infection or illness, and headaches can arise from numerous causes, including tension, migraines, or other health issues unrelated to allergies. Thus, the presence of hives serves as a clear indicator that an allergic reaction is occurring.