A 19-year-old man develops recurrent episodes of laryngeal edema and abdominal pain without urticaria. Complement studies show low C4 levels with normal C3, and assays reveal deficient functional activity of C1 esterase inhibitor. The mediator responsible for the angioedema is predominantly:

• A Histamine
• B Prostaglandin D2
• C Leukotriene D4
• D Bradykinin ✓

Explanation

In hereditary angioedema, lack of C1 inhibitor permits uncontrolled activation of the contact system, generating excess bradykinin, which increases vascular permeability and produces the edema. The absence of urticaria and pruritus argues against a histamine-mediated allergic reaction, and antihistamines are ineffective. Low C4 with normal C3 reflects classical pathway consumption distal to C1, confirming the diagnosis biochemically.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.