A 25-year-old man presents with recurrent episodes of non-pruritic, non-urticarial swelling of the face and extremities. C1 inhibitor level is found to be markedly decreased. Which complement component is most likely to be chronically low in this condition due to continuous classical pathway activation?

• A C3
• B C9
• C C5
• D C4 ✓

Explanation

Hereditary angioedema results from C1 inhibitor deficiency. Without C1 inhibitor, the classical pathway is continuously activated, leading to unregulated consumption of C4 and C2. C3 levels are typically normal because the alternative pathway is not primarily involved, and C5-C9 are downstream and less affected.

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.