Which symptom would indicate a hypersensitivity reaction to penicillin?

Urticaria, commonly known as hives, is a classic indicator of a hypersensitivity reaction, particularly to penicillin. This condition is characterized by raised, itchy welts on the skin which occur due to the release of histamines and other inflammatory mediators in response to an allergen, such as penicillin. Hypersensitivity reactions can manifest in various forms, but urticaria is directly linked to the immune system's overreaction to the antibiotic.

Fever can occur for a variety of reasons including infections or other reactions but is not specific to hypersensitivity. Headaches and abdominal cramps may also arise from many causes unrelated to allergic reactions, making them less specific indicators for such hypersensitivity to penicillin. Therefore, the presence of urticaria specifically points to an allergic or hypersensitivity response, distinguishing it as the optimal choice in this scenario.