This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Medicine
[{'content': 'Adenosine deaminase deficiency is seen in the following: \nSelect and explain. \nA. Common variable immunodeficiency, B. Severe combined immunodeficiency, C. Chronic granulomatous disease, D. Nezelof syndrome', 'role': 'user'}, {'content': "Answer: B \nExplanation: The most common cause of autosomal recessive severe combined immunodeficiency(SCID) is the deficiency of enzyme adenosine deaminase (ADA)Defect 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 cases(ref: Harrison's 18/e p2700)", 'role': 'assistant'}]