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
Adenosine deaminase deficiency is seen in the following: 
Select and explain. 
A. Common variable immunodeficiency, B. Severe combined immunodeficiency, C. Chronic granulomatous disease, D. Nezelof syndrome
B: Severe combined immunodeficiency
Answer: B: Severe combined immunodeficiency 
Explanation: 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)