A 4-year-old boy presents with a 2-week history of fatigue, intermittent low-grade fevers, and bone pain that awakens him at night. Physical exam reveals pallor, diffuse petechiae, and hepatosplenomegaly. A complete blood count shows anemia, thrombocytopenia, and a white blood cell count of 3,500/μL with atypical lymphocytes. What is the most common immunophenotype for this likely diagnosis?