This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Primary Immunodeficiency, Subject name: Pediatrics
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: Isolated IgA Deficiency: commonest immunodeficiency (1/600) Affected individuals have extremely low levels of both serum and secretory IgA C/F: asymptomatic, recurrent respiratory infections