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A 21-year-old man has a productive cough, wheeze and steatorrhoea. On examination he is clubbed and cyanosed, and has bilateral coarse crackles. His blood glucose level is measured at 11 mmol/L.

L Cystic fibrosis is an autosomal recessive condition associated with a mutation in the CFTR (cystic fibrosis transmembrane conductance regulator) gene on chromosome 7. Patients are susceptible to recurrent respiratory infection and the development of bronchiectasis. Acute exacerbations are often caused by Pseudomonas spp. which may be highly resistant to antibiotics. Haemoptysis is common and may indicate the presence of aspergilloma. Pancreatic insufficiency usually develops resulting in malabsorption, steatorrhoea and development of diabetes mellitus. Growth and puberty are delayed in most patients. Males are usually infertile as a result of the failure of the vas deferens and epididymis to develop. High sweat sodium and chloride concentrations (>60 mmol/L) are highly suggestive of the disease.