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': 'Commonest immunoglobulin deficiency is of: \nSelect and explain. \nA. IgA, B. IgE, C. IgG, D. IgD', 'role': 'user'}, {'content': 'Answer: A \nExplanation: IgA', 'role': 'assistant'}]