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
A boy complains of bleeding gums, swollen, Joints with hemorrhage into joints. His paternal and maternal uncle complains of same problem. It is due to deficiency of factor: 
Select and explain. 
A. VIII, B. IX, C. X, D. VI
A: VIII
Answer: A: VIII 
Explanation: None