A 24-year-old male was stung by a bee. He has urticaria and redness on the chest and neck, with hypotension (BP 80/20). What is the mechanism behind his hypotension?

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In an acute allergic reaction like a bee sting leading to anaphylaxis, a burst of mediators from mast cells and basophils causes two major hemodynamic changes: widespread vasodilation and increased capillary permeability. The resulting fluid and protein leakage from the intravascular space into the interstitial tissues lowers the circulating blood volume, which drops arterial pressure. The swelling, redness, and urticaria you see are signs of the capillary leak, while the hypotension comes from this reduced effective circulating volume combined with vasodilation.

So the key mechanism is increased capillary permeability causing plasma to leak into interstitial spaces (third-spacing), contributing to the low blood pressure. Increased capillary leak is what best explains the hypotension in this scenario; decreased permeability would lessen leakage, direct myocardial depression is not the primary driver in typical anaphylaxis, and vascular spasm would tend to raise, not lower, blood pressure.