SCID is caused due to deficiency of?

ANSWER: (C) Adenosine deaminaseREF: Lippincott s biochemistry 4th Ed page 301ADENOSINE DEAMINASE (ADA) DEFICIENCY: ADA is expressed in the cytosol of all cells, but, in humans, lymphocytes have the highest activity of this enzyme. A deficiency of ADA results in an accumulation of adenosine, which is converted to its ribonucleotide or deoxyribonucleotide forms by cellular kinases. As dATP levels rise, ribonucleotide reductase is inhibited, thus preventing the production of all deoxyribose-containing nucleotides. Consequently, cells cannot make DNA and divide. In its most severe form, this autosomal recessive disorder causes a type of severe combined immunodeficiency disease, involving a decrease in both T cells and B cells. It is estimated that in the United States, ADA deficiency accounts for approximately 14% of all cases of SCID. Treatment requires either bone marrow' replacement or enzyme replacement therapy. Without appropriate treatment, children with this disorder usually die by the age of two.