A 29-year-old woman with no significant past medical history presents with recurrent episodes of an intensely pruritic rash for the past 3 months. She reports that the lesions appear spontaneously 2–3 times per day and typically resolve within 6–8 hours without leaving bruising or pigmentation. She cannot identify a consistent trigger. Physical exam during an episode shows multiple well-circumscribed, raised erythematous plaques on the trunk and proximal extremities. No fever, arthralgias, or abdominal pain are reported. Which of the following is the most appropriate first-line pharmacotherapy for this condition?