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. Genetic testing for CFTR mutations is incorrect as the primary diagnostic tool. While genetic testing can identify specific CF mutations and is useful for screening or confirming atypical cases, it is not considered the first-line gold standard for diagnosis. B. Sweat chloride test is correct. The sweat chloride test measures the concentration of chloride in sweat, which is abnormally elevated in individuals with cystic fibrosis due to defective CFTR channels. A chloride concentration ≥60 mmol/L on two separate occasions confirms the diagnosis. It remains the gold standard diagnostic test for CF in children. C. Chest x-ray is incorrect because it is not diagnostic for CF. Although chest x-rays may show structural lung changes such as hyperinflation or bronchiectasis in advanced disease, they cannot confirm CF on their own. D. Pulmonary function tests (PFTs) are incorrect for initial diagnosis in young children. PFTs assess lung function and disease progression but are not reliable for confirming CF, especially in a 3-year-old who may not be able to perform the maneuvers required.