Cystic fibrosis and primary ciliary dyskinesia Quiz: Ace Your Exams

A 6-year-old girl presents to the pediatrician for evaluation of recurrent sinusitis and pulmonary infections. She has been frequently sick and missing school, and the parents are concerned. She has a history of chronic constipation, and her parents report she has large stools that float in the toilet. She is at the 14^th percentile for height and the 4^th percentile for weight. Temperature is 37.1°C (98.8 F), pulse is 106/min, respirations are 24/min, blood pressure is 110/72 mmHg, and oxygen saturation is 99%. On physical examination, the patient appears thin but is otherwise interactive and playful. Pulmonary exam reveals mild diffuse rhonchi without increased work of breathing. Heart rate is regular with no murmur appreciated. The patient is referred for a sweat test which reveals a sweat chloride level of 68 mmol/L. Which of the following should be ordered next to confirm the diagnosis?