This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Christmas disease is due to deficiency of: September 2010 \nSelect and explain. \nA. Deficiency of factor IX, B. Deficiency of factor X, C. Deficiency of factor XI, D. Deficiency of factor XII', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A: Deficiency of factor IX Haemophilia B (or hemophilia B) is a blood clotting disorder caused by a mutation of the Factor IX gene, leading to a deficiency of Factor IX. It is the less common form of haemophilia, rarer than haemophilia A. It is sometimes called Christmas disease after Stephen Christmas, the first patient described with this disease', 'role': 'assistant'}]