This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'Which of the following is the most common form of primary immunodeficiency? \nSelect and explain. \nA. AIDS, B. Wiskott-Aldrich syndrome, C. Common variable immunodeficiency, D. Selective IgA immunodeficicy', 'role': 'user'}, {'content': "Answer: D \nExplanation: Selective IgA deficiency (less than 15mcg/ml) is the most common type of primary immunodeficiency. Individuals develop recurrent acute and chronic respiratory infections that may lead to bronchiectasis. These patients are at an increased risk of drug allergies, atopic disorders and autoimmune diseases. Ref: Harrison's Principles of Internal Medicine, 18th edition, chapter 316.", 'role': 'assistant'}]