This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunodeficiency Disorders, Subject name: Pathology
A 7-month-old boy has had multiple bouts of otitis media, sinusitis, bronchitis, oral candidiasis, and multiple viral infections. Cessation of the recurrent infections follows successful engraftment of a bone marrow transplant. The basis of the clinical improvement is 
Select and explain. 
A. direct transfusion of antibodyproducing B cells., B. direct transfusion of donor CD4+ and CD8+ lymphocytes., C. donor suppression of recipient cytotoxic T cells, D. maturation of donor lymphoid progenitor cells
D: maturation of donor lymphoid progenitor cells
Answer: D: maturation of donor lymphoid progenitor cells 
Explanation: Severe combined immunodeficiency disease(SCID) Characterized by failure to thrive and increased susceptibility to bacterial, fungal, and viral infections. Laboratory studies reveal decreased numbers of both B cells and T cells and deficiency of immunoglobulins. The treatment of choice is bone marrow (or other sources of hematopoietic stem cells) transplantation and is based on maturation of donor lymphoid progenitor cells