What is a hallmark finding of chronic myelogenous leukemia (CML)?

The hallmark finding of chronic myelogenous leukemia (CML) is the presence of the Philadelphia chromosome in leukemic cells. The Philadelphia chromosome results from a translocation between chromosomes 9 and 22, which creates the BCR-ABL fusion gene. This fusion gene encodes a tyrosine kinase that contributes to the uncontrolled proliferation of myeloid cells characteristic of CML.

The detection of the Philadelphia chromosome is crucial in diagnosing CML and helps distinguish it from other hematological disorders, making it a specific and significant marker for this type of leukemia. It also has implications for targeted therapy, as treatments such as tyrosine kinase inhibitors specifically target the effects of the BCR-ABL fusion protein, improving patient outcomes.

The other options may be associated with different conditions or stages of leukemia but are not specific to CML. For example, pancytopenia with circulating blasts can suggest acute leukemia rather than chronic. Noncirculating blast cells in the bone marrow are more indicative of certain acute leukemias or other conditions. Lymphocytosis is often seen in chronic lymphocytic leukemia (CLL) and not typically in CML. Thus, the Philadelphia chromosome serves as a definitive marker that guides both diagnosis and treatment.