A 16-year-old boy has recurrent episodes of nonpruritic angioedema of the face, larynx, and bowel wall causing colicky pain, unresponsive to antihistamines and corticosteroids. C4 levels are persistently low during and between attacks. The deficient protein responsible for this condition normally inhibits:

• A C1r and C1s esterases, factor XII, and kallikrein ✓
• B Factor Xa and thrombin
• C C5 convertase only
• D Plasminogen activator inhibitor

Explanation

Hereditary angioedema results from C1 inhibitor (C1-INH) deficiency. C1-INH is a serine protease inhibitor that controls activation of C1r/C1s, factor XII, and kallikrein; its absence permits uncontrolled bradykinin generation, producing the edema. Low C4 from ongoing classical pathway consumption is the screening clue. Antihistamines fail because histamine is not the mediator, distinguishing it from allergic angioedema.

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.