<prompt> Adenosine deaminase deficiency is seen in the following:Common variable immunodeficiencySevere combined immunodeficiencyChronic granulomatous diseaseNezelof syndrome  <response>: The most common cause of autosomal recessive severe combined immunodeficiencySCID is the deficiency of enzyme adenosine deaminase ADADefect lies in lymphoid stem cells, resulting in failure of both T and B cellsDeficiency of ADA leads to accumulation of deoxyadenosine its derivatives, which are toxic to immature lymphocytes, especially those of T lineage, and this may account for the marked decrease in T lymphocyte number in severe casesref: Harrisons 18e p2700 