A nurse is caring for a client who has urticaria following administration of an antibiotic. Which of the following medications should the nurse prepare to administer?

A. Diphenhydramine: Diphenhydramine is an H1 receptor antagonist that inhibits the binding of histamine to H1 receptors in the skin and vasculature. By blocking histamine-mediated vasodilation, increased capillary permeability, and sensory nerve stimulation, it reduces the erythema, pruritus, and edema associated with urticaria. B. Hydralazine: Hydralazine is a direct-acting arteriolar vasodilator used to reduce systemic vascular resistance in hypertensive crises or chronic hypertension. It has no effect on histamine pathways or IgE-mediated allergic responses and therefore does not alleviate urticaria or other manifestations of hypersensitivity reactions. C. Protamine: Protamine sulfate is a cationic protein used to neutralize unfractionated heparin by forming a stable complex, thereby reversing anticoagulation. It does not interfere with histamine release, IgE receptor binding, or any pathophysiologic mechanisms underlying allergic reactions, making it irrelevant for urticaria management. D. Naloxone: Naloxone is a competitive opioid receptor antagonist that reverses opioid-induced respiratory depression by displacing agonists at the mu-opioid receptor. It has no role in modulating histamine release or the immunologic processes that produce urticaria, and administration would not alleviate allergic symptoms.