A 2-month-old infant presents with recurrent pneumonia and oral thrush. The parents report poor weight gain and persistent diarrhea. Physical examination reveals a lack of tonsils and palpable lymph nodes. Laboratory results show low T-cell, B-cell, and NK-cell counts. The patient is diagnosed with adenosine deaminase (ADA) deficiency. Which of the following is a key pathological feature in this form of SCID?
