(a) Both the girls A and B are suffering from rare genetic disorder of adenosine deaminase deficient severe combined immunodeficiency disease (ADA–SCID). In the absence of this enzyme, toxic levels of the phosphorylated form of its substrate, deoxyadenosine, accumulates in T lymphocytes (white blood cells essential to an immune response) and kill them
(b) ADA deficiency can be treated either by bone marrow transplantation or by enzyme replacement therapy (ERT) by giving an intravenous injection of ADA. However, both approaches are not completely curative as these lymphocytes are not immortal and require repeated treatment. Girl A must have been treated with one of these therapies.
(c) Girl B must have been treated using gene-therapy where the gene isolate from marrow cells producing ADA was introduced into cells at an early embryonic stage for a possible permanent cure.