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. A stool fat content analysis can suggest malabsorption issues but does not specifically confirm cystic fibrosis. B. The sweat chloride test measures the amount of chloride in the sweat, with elevated levels confirming a diagnosis of cystic fibrosis. C. Pulmonary function tests assess lung function but are not definitive for diagnosing cystic fibrosis. D. A sputum culture can identify respiratory infections but is not specific for cystic fibrosis diagnosis.