Which of the following diseases is not classified under types of leukemia?

Pernicious anemia is characterized by a deficiency of vitamin B12 due to the inability of the body to absorb it properly. This condition primarily affects red blood cell production, leading to a specific type of anemia, rather than being related to the abnormal proliferation of blood cells that defines leukemia.

Leukemia is a group of cancers that typically arise from the bone marrow and result in the overproduction of abnormal white blood cells. Acute lymphoblastic leukemia, chronic lymphocytic leukemia, and chronic myeloid leukemia all fall under the classification of leukemia as they involve different types of white blood cells and exhibit distinct clinical features and progression patterns.

Thus, while pernicious anemia is a serious blood condition, it does not share the characteristics of leukemia, making it the correct choice for this question.