Which physical exam finding is indicative of an allergic reaction?

Edema is a hallmark sign of an allergic reaction and often occurs due to the release of inflammatory mediators such as histamine from mast cells and basophils. When these mediators are released in response to an allergen, they increase vascular permeability, leading to the accumulation of fluid in surrounding tissues, which manifests as swelling or edema. This localized swelling can occur in various parts of the body, such as the skin (urticaria or hives) or mucosal tissues (angular cheilitis or conjunctivitis), when an individual is exposed to an allergen.

In contrast, fever is typically associated with infections and systemic inflammatory responses rather than allergic reactions. A non-blanching rash, while notable, usually suggests conditions such as vasculitis or petechiae, which are not exclusive to allergic reactions. Cyanosis indicates a lack of oxygen in the blood, often related to respiratory or cardiac issues, rather than an immune response to allergens. Therefore, the presence of edema distinctly signifies an allergic response.