Answer is B (Demonstrate an abnormal nasal potential difference) Nasal Potential Difference is a sensitive test of electrolyte transpo (CFTR function) that can be used to suppo or refute a diagnosis of cystic fibrosis when sweat chloride levels are normal or border line and two CF mutations cannot be demonstrated by DNA testing. Diagnostic criteria for cystic fibrosis Risk Factor For CF PLUS Laboratory Evidence of CFTR Dysfunction * Presence of one or more typical clinical feature * Positive sweat chloride test (> 1 phenotypic symptom) Plus (>60 mcq/l on two occasions) or Or * History of CF in sibling Plus * Identification of 2 CF mutations or or * Positive Neonatal screening test Plus * Abnormal Nasal Potential Difference The patient in question has typical clinical features of cystic fibrosis; however the sweat chloride levels are border line (between 40-60meq/1) on two separate occasions. To establish the diagnosis of cystic. fibrosis, another laboratory evidence demonstrating CFTR dysfunction is therefore required. This can be achieved by demonstration of abnormal CF mutation by DNA analysis or by demonstrating an abnormal nasal potential difference. The diagnostic criteria for cystic fibrosis requires demonstration of at least two CF mutations and hence demonstration of an abnormal F508 mutation alone is not sufficient to establish the diagnosis of CF An abnormal CFTR nasal potential difference is an established laboratory evidence of CFTR dysfunction and is accepted as a diagnostic criterion to establish the diagnosis of cystic fibrosis.