A 6-year-old girl from rural India presents with a 2-week history of fever, bone pain, and easy bruising. Investigations reveal WBC 8,500/µL with 85% blasts, hemoglobin 7.2 g/dL, and platelets 45,000/µL. Flow cytometry shows CD19+, CD10+, TdT+, MPO−. Which single feature most reliably distinguishes common B-cell ALL (cALL) from T-cell ALL (T-ALL) in this age group?