A 5-year-old girl from Mumbai presents with a 2-month history of progressive pallor, fatigue, and recurrent infections. Her parents report she had a viral illness 4 weeks ago. On examination, she has generalized lymphadenopathy, hepatomegaly (2 cm below costal margin), and splenomegaly (3 cm). Complete blood count shows hemoglobin 6.8 g/dL, WBC 120,000/μL with 80% lymphoblasts, and platelets 28,000/μL. Bone marrow shows >95% lymphoblasts that are CD19+, CD10+, and TdT+. What is the most likely cytogenetic abnormality associated with the best prognosis in this case?