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Bronchiectasis is dilation and destruction of larger bronchi caused by chronic infection and inflammation. Bronchiectasis may be diffuse or focal. In developed countries, many cases of diffuse bronchiectasis appear initially to be idiopathic. With newer, improved genetic and immunologic testing, an increasing number of reports describe finding an etiology in these idiopathic cases after careful, systematic evaluation. Which of the following is the most common identified cause of diffuse bronchiectasis?