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?

Rationale: A. The sweat chloride test is the gold standard for diagnosing cystic fibrosis. A chloride concentration ≥ 60 mEq/L confirms the diagnosis. B. A sputum culture may identify bacterial colonization (e.g., Pseudomonas aeruginosa) but does not confirm CF. C. Pulmonary function tests help determine the extent of lung disease but are not diagnostic of CF. D. Stool fat analysis may indicate malabsorption, which is common in CF, but it does not confirm the diagnosis.