Before starting antimicrobial therapy, the nurse assesses a patient for possible drug allergy. Which of the following complaints should alert the nurse to a drug allergy?

The correct answer is B: Hives or shortness of breath. These symptoms are common signs of an allergic reaction to a drug. Hives indicate an immune response causing skin rash, while shortness of breath suggests possible respiratory distress, which can be life-threatening in severe cases. Headaches (A), diarrhea (C), and nausea (D) are common side effects of medications but not specific indicators of an allergic reaction. In summary, hives or shortness of breath are crucial signs of a drug allergy due to their association with immune system response and potential severity compared to other symptoms.