A 25-year-old receives penicillin and develops urticaria, bronchospasm, and hypotension within minutes. The mechanism is IgE-mediated mast cell degranulation. The vasoactive mediator released first (preformed, from granules) causing immediate vasodilation is:

• A Histamine ✓
• B Prostaglandin D2
• C Leukotriene C4
• D Platelet-activating factor

Explanation

Histamine is the principal preformed mediator stored in mast cell and basophil granules; it is released immediately upon IgE cross-linking and causes vasodilation, increased vascular permeability, smooth muscle contraction, and increased mucus secretion. Prostaglandin D2 and leukotrienes are newly synthesized from arachidonic acid after mast cell activation (secondary phase). PAF is also synthesized de novo and contributes to the late phase.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.