This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Primary Immunodeficiency, Subject name: Pediatrics
"Hereditary angioneurotic edema" is due to deficiency of: 
Select and explain. 
A. C1 inhibitor, B. S protein, C. C3b inactivator, D. C2 kinins
A: C1 inhibitor
Answer: A: C1 inhibitor 
Explanation: Hereditary angioneurotic edema occurs due to deficient or defective C1 inhibitor Protein S deficiency- | risk of venous thrombosis