Which diagnostic test confirms cystic fibrosis?

Choice A rationale The sweat chloride test is the gold standard diagnostic test for cystic fibrosis (CF). CF is caused by a mutation in the CFTR gene, which impairs chloride ion transport across epithelial cells. This defect leads to abnormally high chloride concentration in sweat, typically >60 mEq/L in children, which provides biochemical confirmation of the disease. Choice B rationale Pulmonary function tests (PFTs) measure lung volumes and airflow, typically revealing an obstructive pattern in CF due to chronic infection and inflammation. While they monitor disease progression and severity, PFTs are not diagnostic for the underlying genetic disorder that defines cystic fibrosis. Choice C rationale A Chest X-ray (CXR) may show characteristic findings of chronic lung disease in CF, such as bronchiectasis, hyperinflation, or atelectasis, reflecting the disease's pulmonary complications. However, the CXR is an imaging tool for assessing lung structure, not a diagnostic test for the systemic genetic condition itself. Choice D rationale Sputum cultures are used to identify the specific bacterial pathogens, such as Pseudomonas aeruginosa or Staphylococcus aureus, colonizing the airways of CF patients. While essential for guiding antibiotic therapy, a sputum culture only detects secondary infection and does not confirm the diagnosis of cystic fibrosis.