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. Stool fat content analysis: This test may suggest fat malabsorption seen in cystic fibrosis, but it is not diagnostic. It supports the presence of pancreatic insufficiency. B. Sputum culture: While useful for identifying specific infections (e.g., Pseudomonas), it does not diagnose CF. C. Sweat chloride test: This is the most definitive diagnostic test for CF. A chloride concentration >60 mEq/L is considered diagnostic in children. D. Pulmonary function test: This test assesses lung function but is not used to diagnose CF. It is more useful in monitoring disease progression.