Final answer:
A 3-year-old girl showing signs of ALL should be tested with a bone marrow biopsy. Treatment involves chemotherapy and supportive care, with certain factors indicating a poorer prognosis.
Step-by-step explanation:
The clinical presentation of a 3-year-old girl with a limp, leg pain, a slight fever (T = 99.9), hepatosplenomegaly (HSM), petechiae, pallor, and abundant lymphoblasts on her blood smear, which are CALLA and TdT positive, points towards a diagnosis of acute lymphoblastic leukemia (ALL). The best test to confirm this diagnosis would be a bone marrow biopsy to evaluate for the presence of leukemic blast cells, which characteristically infiltrate the bone marrow in ALL. Treatment for ALL typically involves two main strategies: chemotherapy and supportive care, which includes management of any complications like infections. Known poor prognostic factors in ALL include a high white blood cell count at diagnosis, certain genetic abnormalities, age younger than 1 or older than 10 years at diagnosis, and the presence of CNS or testicular involvement at diagnosis.