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
Richter's syndrome refers to which of the following malignant transformation? 
Select and explain. 
A. CLL evolving into aggressive lymphoma, B. Hairy cell leukemia evolving to AML, C. Blast crisis in CML, D. Splenic infiltration in NHL
A: CLL evolving into aggressive lymphoma
Answer: A: CLL evolving into aggressive lymphoma 
Explanation: Richter's transformation or Richter's syndrome is a complication of B cell chronic lymphocytic leukemia (CLL) or hairy cell leukemia (HCL) in which the leukemia changes into a fast-growing diffuse large B cell lymphoma.