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
[{'content': '"Hereditary angioneurotic edema" is due to deficiency of: \nSelect and explain. \nA. C1 inhibitor, B. S protein, C. C3b inactivator, D. C2 kinins', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Hereditary angioneurotic edema occurs due to deficient or defective C1 inhibitor Protein S deficiency- | risk of venous thrombosis', 'role': 'assistant'}]