Which finding is most characteristic of chronic myelogenous leukemia?

• A. Usually presents in children
• B. Absence of the Philadelphia chromosome
• C. Philadelphia chromosome positive
• D. Low white blood cell count

Answer: (C)

Chronic myelogenous leukemia is defined by a specific genetic change: the Philadelphia chromosome, produced by a t(9;22) translocation that creates the BCR-ABL fusion gene with constitutive tyrosine kinase activity. This abnormal signaling drives uncontrolled proliferation of myeloid cells, leading to a very high white blood cell count with a spectrum of mature to immature granulocytes and often basophilia. The presence of the Philadelphia chromosome is the hallmark finding that distinguishes CML from other hematologic conditions and is detectable in most patients. The other statements don’t fit the typical picture: this disease most often presents in adults, not children; it features leukocytosis rather than a low white blood cell count; and absence of the Philadelphia chromosome would be inconsistent with CML’s defining feature.