What is a wheal?

A wheal is specifically defined as a raised, swollen, well-defined area on the skin, which is often associated with an allergic reaction or a skin condition such as hives. This swelling occurs due to the accumulation of fluid in the dermis, usually as a result of the release of histamine and other chemicals from mast cells in response to an allergen or irritant. The clear borders of a wheal distinguish it from other skin reactions, as it is typically localized rather than widespread.

In contrast, the other options describe different conditions or reactions that do not fit the definition of a wheal. Localized swelling involving the lips, tongue, and larynx pertains to angioedema and is usually more serious, potentially leading to airway obstruction. Generalized itching with small raised areas would relate more to a rash or urticaria but lacks the specific definition of a wheal. An exaggerated immune response to any substance encompasses a broader concept, including anaphylaxis, which is not limited to the skin manifestations characteristic of a wheal. Thus, option A clearly captures the essence of what a wheal is, making it the correct choice.