82. A 17 year old is seen for chronic nasal obstruction. She reports inability to breathe through her nose for several months. She has a history of recurrent bronchitis, sinusitis, and chronic cough. Review of systems is pertinent for bulky stools with a greasy film left in the toilet. The family history is noncontributory. On physical examination, her weight is at the 80th percentile and body mass index is at the 78th percentile. There is bilateral nasal obstruction with polypoid material filling both antra. Her oropharynx is clear. Chest examination is notable only for a few inspiratory crackles in the left upper chest posteriorly; no wheezing is heard. Point of maximum cardiac impulse is in the left midclavicular line. Her abdomen is soft and there is no hepatosplenomegaly. There is no digital clubbing. Serum IgE values are normal and radioallergosorbent test for multiple inhalant antigens shows negative results. Pulmonary function testing shows a mild combined restrictive and obstructive pattern with no bronchodilator response. Of the following, the MOST appropriate next diagnostic step is a. CFTR mutation analysis b. computed tomography of the sinuses c. quantitative immunoglobulin determination d. sweat chloride measurement