The best indicator of cystic fibrosis is: (A) A positive family history of cystic fibrosis, (B) The presence of digital clubbing, (C) A sweat test with a chloride concentration of 70mEq/L, (D) Bronchiectasis on a chest x-ray
Answer is C. A positive sweat test (i.e., a sweat chloride level greater than 60 mEq/L) is diagnostic of cystic fibrosis.