A nurse is reinforcing teaching about diagnostic tests with the parents of a child who has suspected cystic fibrosis. Which of the following diagnostic tests should the nurse include as the most definitive when diagnosing cystic fibrosis?

A. Pulmonary function test: While useful in assessing lung function, this test is not definitive for diagnosing cystic fibrosis. B. Sputum culture: This can help identify respiratory infections, but it is not used for diagnosing cystic fibrosis. C. Sweat chloride test: The sweat chloride test is the most definitive diagnostic test for cystic fibrosis. Elevated chloride levels in the sweat are indicative of the disease. D. Stool fat content analysis: This test can be helpful in identifying malabsorption issues but is not definitive for cystic fibrosis diagnosis.