This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
The commonest Primary immunodeficiency is - 
Select and explain. 
A. Common variable immunodeficiency, B. Isolated IgA immunodeficiency, C. Wiskott-Aldrich syndrome, D. AIDS
B: Isolated IgA immunodeficiency
Answer: B: Isolated IgA immunodeficiency 
Explanation: Selective immunoglobulin A (IgA) deficiency (SIgAD) is a genetic immunodeficiency, a type of hypogammaglobulinemia. People with this deficiency lack immunoglobulin A (IgA), a type of antibody that protects against infections of the mucous membranes lining the mouth, airways, and digestive tract. It is defined as an undetectable serum IgA level in the presence of normal serum levels of IgG and IgM, in persons older than 4 years. It is the most common of the primary antibody deficiencies. Most such persons remain healthy throughout their lives and are never diagnosed.