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: Medicine
What percentage of the MDR TB cases are XDR TB, worldwide? 
Select and explain. 
A. 5%, B. 10%, C. 20%, D. 25%
B: 10%
Answer: B: 10% 
Explanation: Probably 10% of the MDR-TB cases worldwide are XDR-TB (extended drug resistant) Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 1341