Which of the following best describes a sign of an allergic reaction to medication?

Hives, also known as urticaria, are raised, itchy welts on the skin that often indicate an allergic reaction. When the body’s immune system responds to a medication or allergen, it can trigger the release of histamines, which contribute to the development of hives. This skin response is a classic and observable sign of an allergy, making it easily identifiable during a physical examination.

In contrast, other options like a rapid heartbeat, severe headache, and muscle cramps may occur for various reasons, including anxiety, drug side effects, or other health conditions, but they are not specific signs of an allergic reaction. Hives directly demonstrate the body's hypersensitivity and immune response to an allergen, emphasizing their significance in identifying allergic reactions.