<prompt> The best indicator of cystic fibrosis isA positive family history of cystic fibrosisThe presence of digital clubbingA sweat test with a chloride concentration of 70mEqLBronchiectasis on a chest x-ray  <response>: A positive sweat test i.e., a sweat chloride level greater than 60 mEqL is diagnostic of cystic fibrosis. 