What type of leukemia is characterized by immature lymphocytes and has a rapid onset?

Acute Lymphocytic Leukemia (ALL) is characterized by the rapid production of immature lymphocytes, known as lymphoblasts. This type of leukemia has a swift onset and is primarily seen in children, though it can also occur in adults. The classification of ALL as "acute" indicates that the disease progresses quickly and requires prompt diagnosis and treatment.

The presence of immature lymphocytes is significant because these cells are not fully developed, which impairs the immune system's ability to function effectively. This lack of maturity leads to symptoms such as fatigue, frequent infections, and easy bruising or bleeding, ultimately necessitating immediate medical intervention. The other terms mentioned do not fit this description; for instance, Chronic Myeloid Leukemia is a slower-growing form of cancer that typically involves more mature cells, and Polycythemia Vera is a different condition entirely related to increased red blood cell production. Lymphocytosis indicates an increase in lymphocytes but is not a type of leukemia by itself. Thus, the defining features of rapid onset and the presence of immature lymphocytes distinctly categorize Acute Lymphocytic Leukemia as the correct answer.