This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
[{'content': 'The best indicator of cystic fibrosis is \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: C \nExplanation: A positive sweat test (i.e., a sweat chloride level greater than 60 mEq/L) is diagnostic of cystic fibrosis.', 'role': 'assistant'}]