<s>[INST] context: Patients with cystic fibrosis are now surviving into adulthood. The median survival exceeds age 40. Most cases are diagnosed in childhood; however, because of variable penetration of the genetic defect, approximately 7% are not found until the patient is an adult. Most male patients (>95%) are azoospermic. Chronic pulmonary infections occur, and bronchiectasis frequently develops. Diabetes mellitus and gastrointestinal problems indicate pancreatic insufficiency. This patient should have sweat chloride measurement; if abnormal (sweat Cl above 70 mEq/L), cystic fibrosis transmembrane conductance regulator (CFTR) mutation analysis should be ordered. COPD or emphysema at this age would be unusual unless the patient were deficient in alpha- 1 antitrypsin. Immunoglobulin deficiencies can cause recurrent sinopulmonary infections but would not cause malabsorption or infertility. Whipple disease causes malabsorption but not the pulmonary manifestations or infertility; it would be vanishingly rare in a young patient. Asthma would not cause the abdominal symptoms, diabetes, or changes of bronchiectasis. [/INST] question: A 25-year-old man presents to the clinic for evaluation of infertility. He has a lifelong history of a productive cough and recurrent pulmonary infections. On his review of symptoms he has indicated chronic problems with abdominal pain, diarrhea, and difficulty gaining weight. He also has diabetes mellitus. His chest x-ray suggests bronchiectasis. Which is the most likely diagnosis? | options: A: Chronic obstructive pulmonary disease (COPD) B: Immunoglobulin deficiency C: Cystic fibrosis D: Whipple disease | answer: C </s>