A patient develops widespread urticaria and laryngeal oedema 20 minutes after IV injection of iodinated contrast. Which pathophysiological mechanism is PRIMARILY responsible for this reaction?

• A Anaphylactoid (non-IgE-mediated) direct mast cell and complement activation ✓
• B IgE-mediated Type I hypersensitivity (anaphylaxis)
• C Type III immune complex-mediated hypersensitivity
• D Direct osmotic renal tubular toxicity

Explanation

Acute hypersensitivity-like reactions to iodinated contrast media are predominantly anaphylactoid (pseudoallergic) — they mimic anaphylaxis clinically but are NOT IgE-mediated. The mechanism involves direct non-immunological activation of mast cells and basophils, complement activation, and bradykinin release by the contrast molecule. True IgE-mediated anaphylaxis to contrast is rare. This distinction is clinically important because skin testing for IgE allergy is unreliable and premedication with corticosteroids/antihistamines is used for prevention rather than desensitisation.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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