Which symptom may indicate a penicillin allergy in patients?

A rash on the arm is a notable symptom that may indicate a penicillin allergy. Allergic reactions to penicillin can manifest in various forms, with skin reactions such as rashes being quite common. These rashes can range from mild to severe and typically appear within a few hours to a few days after exposure to the medication.

In the context of allergy responses, rashes are particularly significant because they signal the immune system's reaction to the penicillin. This condition can sometimes progress to more serious manifestations, such as hives or anaphylaxis, which further highlights the importance of recognizing and documenting such symptoms in patients with a suspected penicillin allergy.

The other symptoms listed, while potentially related to other health issues, are not specific indicators of a penicillin allergy. For instance, dry mouth does not correlate with allergic reactions, and stomach pain or headache can have numerous non-allergic causes. Hence, the arm rash stands out as a primary symptom suggestive of a penicillin allergy, warranting close attention and assessment.