Hereditary angioedema is due to deficiency of which protein?

• A. C3
• B. C4
• C. Factor H
• D. C1 esterase inhibitor

Answer: (D)

Hereditary angioedema arises from deficiency or dysfunction of C1 esterase inhibitor, the protein that normally represses the classical complement pathway and the contact system (which leads to bradykinin production). When C1-INH is lacking or dysfunctional, C1 activation proceeds unchecked, causing increased kallikrein activity and excessive bradykinin. This raises vascular permeability and produces angioedema, often without itching or urticaria. C4 is typically consumed and can be low during attacks, reflecting this unregulated activation, but the fundamental defect is the C1 esterase inhibitor deficiency. Other options involve components or regulators of the complement system but do not explain the primary cause of hereditary angioedema.