Cystic fibrosis (CF) is suspected in a toddler. Which test is essential in establishing this diagnosis?

A. Urine creatinine is used to assess kidney function and is not helpful in diagnosing CF. B. The sweat chloride test is the gold standard for diagnosing cystic fibrosis. Elevated chloride levels in sweat are characteristic of CF. C. Serum calcium levels are not specific to CF diagnosis. D. Bronchoscopy is used for assessing airway involvement but is not the primary test for diagnosing CF.