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: Skin
Rx of refractory histiocytosis? 
Select and explain. 
A. Cladarabine, B. High dose MTX, C. High dose cytosine arabinoside, D. Fludarabine
A: Cladarabine
Answer: A: Cladarabine 
Explanation: A. i.e. Cladarabine