A 3-year-old child with a history of recurrent respiratory infections, poor weight gain, and salty-tasting skin is being evaluated for cystic fibrosis (CF). The healthcare provider suspects CF and orders a diagnostic test. Which of the following diagnostic tests is the gold standard for confirming the diagnosis of CF?

A. Sweat chloride test is correct because it is the gold standard diagnostic test for cystic fibrosis. In CF, mutations in the CFTR gene lead to abnormal chloride transport in epithelial cells. This results in elevated chloride levels in sweat, typically ≥60 mmol/L in children, which confirms the diagnosis. B. Genetic testing for CFTR mutations is incorrect as the sole diagnostic tool because not all CF mutations are detectable, and testing may identify carriers or atypical variants. While genetic testing is useful for confirmation or family counseling, it is not the first-line diagnostic test. C. Chest x-ray is incorrect because, although it may reveal hyperinflation, bronchiectasis, or infiltrates indicative of chronic lung disease in CF, it cannot confirm the diagnosis. Imaging is supportive but not diagnostic. D. Pulmonary function tests (PFTs) are incorrect because these tests assess lung function and airflow limitation, but they are not diagnostic for CF, especially in young children who may not be able to perform the tests reliably.