A nurse is caring for a child who has a suspected diagnosis of cystic fibrosis. Which of the following diagnostic tests will confirm the diagnosis?

A. The sweat chloride test is the gold standard for diagnosing cystic fibrosis (CF). A chloride concentration greater than 60 mEq/L is diagnostic for CF in children. B. Pulmonary function tests can assess lung capacity and function, but they are not diagnostic and may be difficult to perform reliably in very young children. C. Stool fat analysis can indicate malabsorption, which is common in CF, but it is not specific or confirmatory for the disease. D. A sputum culture may detect bacterial colonization (e.g., Pseudomonas), which is common in CF, but it does not confirm the diagnosis.