Question
A 6-year-old Caucasian boy is brought to the pediatrician’s office by his mother. She tells you that since he’s been born, he has had multiple bouts of sinusitis. The mother states that his previous pediatrician performed a “sweat test” to help determine the cause of his frequent infections, but that the test came back normal. The boy has no other medical problems. Suspicious of an underlying etiology, his current physician orders nasal transepithelial potential difference measurements. The results show a baseline potential difference that is more negative than normal. Which of the following processes most likely underlies this finding:
| A. |
Increased chloride secretion
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| B. |
Increased sodium absorption
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| C. |
Intracellular potassium depletion
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| D. |
High mucus water content
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Show Answer
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Correct Answer � B
Explanation
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Ans. B. Increased sodium absorption
Explanation:Recurrent sinusitis in a young Caucasian should raise suspicion for cystic fibrosis, as recurrent infections can be caused by the secretion of abnormally thick mucus in the paranasal sinuses. The diagnosis of cystic fibrosis (CF) is usually based on high sweat chloride concentrations, characteristic clinical findings (including sinopulmonary infections), and/or family history. However, a small portion of patients with CF, especially those with “mild” mutations of the CF transmembrane regulator ion channel (CFTR), have near-normal sweat tests.
In these cases, a useful diagnostic adjunct involves measuring the nasal transepithelial potential difference (ie, the charge on the respiratory epithelial surface as compared to the interstitial fluid). Individuals with CF have a significantly more negative nasoepithelial surface than normal, due to increased luminal sodium absorption. The above figure illustrates these abnormalities, applicable to most exocrine glands, but not sweat glands (the CFTR mutation affects sweat glands differently since sweat glands normally use CFTR
channels to absorb luminal sodium chloride). In most exocrine glands, the CFTR protein normally secretes chloride ions into the lumen and also has a tonic inhibitory effect on the opening of the apical sodium channel (which absorbs sodium into the cell). Impaired CFTR functioning directly reduces ductal epithelial chloride secretion and indirectly increases sodium absorption through the lack of CFTR’s inhibitory effect on the apical sodium channel. The result is dehydrated mucus and a widened, negative transepithelial potential difference.
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