What observable symptom would indicate an allergic reaction to medication?

An observable symptom that would indicate an allergic reaction to medication is a skin rash. Allergic reactions often manifest through the skin due to the body's immune response to a perceived foreign substance. When an individual is allergic to a medication, the immune system can trigger various symptoms, with rashes being one of the most common. These rashes can appear as hives, eczema, or other rash formations and are typically red, itchy, and can vary in size and shape.

In contrast, increased appetite and improved energy are not typical signs of an allergic reaction; rather, they might be associated with positive effects of medications or changes in overall health condition. Similarly, a decreased heart rate does not align with allergic reactions and could indicate other health issues. Recognizing that a skin rash signifies an allergic reaction is crucial for prompt intervention and further evaluation of the medication involved.