What is a sign of a severe allergic reaction?

A severe allergic reaction, also known as anaphylaxis, is characterized by physiological changes that significantly threaten a person's health. Difficulty breathing is a critical sign of such a reaction. This symptom results from airway constriction, swelling in the throat, or bronchospasm, which can lead to inadequate oxygen supply and needs immediate intervention. Recognizing difficulty breathing as a sign is crucial because it suggests that the person may not be getting enough air, requiring urgent treatment like epinephrine injection and emergency medical assistance.

In the context of the other options, they represent less severe or different responses. Increased energy generally does not correlate with allergic reactions, and high blood pressure may occur due to stress or anxiety rather than being a direct sign of an allergic reaction. A rash on limbs could indicate a mild allergic response but does not signify the severity required to denote anaphylaxis. Therefore, the focus on difficulty breathing emphasizes the urgent nature of a severe allergic reaction.