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
Examination of a peripheral blood smear demonstrates a leukemia composed of small mature lymphocytes without blast forms. Which of the following is the most likely age of this patient? 
Select and explain. 
A. 1 year, B. 45 years, C. 5 years, D. 65 years
D: 65 years
Answer: D: 65 years 
Explanation: Different leukemias tend to affect populations of different ages. The disease described is chronic lymphocytic leukemia (CLL), which is a disease of older adults. Both the one year-old and the five year-old would be most likely to have acute lymphocytic leukemia (ALL). The 45 year-old would be likely to have either AML or chronic myelogenous leukemia (CML).