The definitive diagnosis of leukemia is made by:

Rationale: A. Bone marrow aspiration with biopsy is the definitive method for diagnosing leukemia. It allows direct examination of marrow cellularity, morphology, and the presence of leukemic blasts, confirming the diagnosis. B. CT scans may detect organ involvement or complications but cannot diagnose leukemia. C. A complete blood count (CBC) can suggest leukemia through abnormal white blood cell counts, anemia, or thrombocytopenia but cannot confirm the diagnosis. D. Lumbar puncture with cerebrospinal fluid analysis is used to assess CNS involvement in leukemia but is not sufficient for initial diagnosis.