A 42-year-old man with a history of seasonal rhinitis and asthma develops acute anaphylaxis 20 minutes after a bee sting. Serum tryptase is elevated. In contrast, a 38-year-old woman with chronic urticaria and angioedema has normal serum tryptase and responds to antihistamines alone, with no history of acute systemic reactions. Which finding best distinguishes mast cell activation in anaphylaxis from chronic urticaria?