Ans. is 'a' i.e., .4denosine deaminase deficiency SODX-Linked recessiveAutosomal recessiveo Most common type of SC ID (50-60%)o Adenosine deaminase deficiencyo Mutation in the common y-chain(.Most common autosomal recessivesubunit of cytokine (IL-2) receptorsform of SCID) o Mutation in JAK - 3 o Mutation in recombinase - activating genes (RAG-1 or RAG-2) o Mutation in Class II MHC moleculeSevere combined immunodeficiencyo Severe combined immunodeficiency (SCID) syndrome is characterized by gross functional impairment of both humoral and cell mediated immunity.o Deficiency of ADA results in overproduction and accumulation of deoxyadenosine which is converted to dAMP, dADP & dATP. dATP is a powerful inhibitor of ribonucleotide reductase,o Thus, it deprives the cell of precursor deoxyribmolecules for DNA synthesis., resulting in apoptosis of lymphocytes.Clinical manifestationso Infants present with prominent thrush (oral candidiasis), extensive diaper rash, and failure to thrive,o Recurrent infection with Candida albicans, P.carinii, Pseudomonas, CMV, varicella and bacteria,o All patients with SCID have very small thymuses( < lgm) that usually fail to descend from the neck, contain no thymocytes and lack corticomeduilary distinction or Hassall corpuscles.o Both the follicular and paracortical areas of the spleen are depleted of lymphocytes,o Lymph nodes, tonsils, adenoids andpeyerspatches are absent or extremely underdeveloped,o Children with SCID rarely survive beyond 1 year and do not tolerate live vaccines.o The only available treatment is bone marrow transplantation. Gene therapy has been successful in X-linked SCID.o In autosomal recessive ( ADA deficiency ) there is rib cage abnormalities simitar to a rachitic rosary and chondroosseous dysplasia, which occur predominantly in the vertebral bodies where a 'bone-in- bone' effect is observed.