What symptom best indicates anaphylaxis in a patient?

The rapid onset of hives, also known as urticaria, is a classic symptom of anaphylaxis and typically occurs shortly after exposure to an allergen. Anaphylaxis is a severe allergic reaction characterized by a range of symptoms that can escalate quickly, affecting multiple body systems. Hives are raised, itchy welts on the skin that indicate the release of histamines and other chemicals in response to an allergen.

In the context of anaphylaxis, hives often appear alongside other symptoms such as difficulty breathing, swelling of the face or throat, and gastrointestinal distress. The rapid onset is particularly significant, as anaphylactic reactions can progress quickly and require immediate medical intervention.

While elevated blood pressure may occur in some situations, it is not a definitive indicator of anaphylaxis, which is more often associated with hypotension due to vascular dilation. Abdominal pain could be part of the reaction but is less specific; it may occur in various medical conditions. Peripheral edema can occur in allergic reactions but is not as immediate or characteristic of anaphylaxis as the appearance of hives. Therefore, the rapid onset of hives is the most telling symptom that indicates a potential anaphylactic reaction, prompting swift assessment and treatment.