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 sputum culture can identify infections associated with cystic fibrosis but does not confirm the diagnosis. B. A stool fat content analysis can indicate malabsorption, a symptom of cystic fibrosis, but it is not diagnostic. C. The sweat chloride test measures the concentration of chloride in sweat and is the gold standard for diagnosing cystic fibrosis. Elevated chloride levels confirm the diagnosis. D. Pulmonary function tests assess lung function and can help manage the disease but are not diagnostic of cystic fibrosis.