A 9-month-old baby boy is brought to your surgery by his mum, who is concerned about his weight gain. His red book measurements have dropped from above the 50th centile to below the 25th centile over the past 4 months. He has had several chest infections, and mum states he frequently has foul-smelling loose stools.

1. Which diagnosis should you be most concerned about?
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In an infant presenting with failure to thrive, recurrent respiratory infections and foul-smelling loose stools, a diagnosis of cystic fibrosis should be suspected.

In infancy and childhood, cystic fibrosis tends to present with the following clinical features:

  • Recurrent chest infections
  • Diarrhoea
  • Failure to thrive
  • Rectal prolapse
  • Nasal polyps
  • Acute pancreatitis
  • Oedema (secondary to hypoproteinaemia)
2. How is this disorder inherited?
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Cystic fibrosis is an autosomal recessive genetically inherited disorder. It is most common in Caucasian populations, affecting approximately 1 in 2500 live births in Europe. 1 in 25 people of European descent carries one allele for cystic fibrosis.

3. What is the cause of this disorder?
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Cystic fibrosis is caused by a defect in a cAMP-regulated channel called the cystic fibrosis transmembrane conductance regulator (CFTR), whose gene is on the long arm of chromosome 7. The commonest gene defect is the D508 mutation, which is a deletion of 3 nucleotides coding for a phenylalanine residue and accounts for approximately 75% of mutations in Caucasians. The CFTR is responsible for regulating the movement of sodium and chloride ions across epithelial membranes, thereby controlling the sodium and chloride concentrations in exocrine secretions. The defective channel results in a high sodium concentration and a low chloride concentration.

4. How can the diagnosis be confirmed?
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Cystic fibrosis can be diagnosed using the following investigations:

  • Immunoreactive trypsin testing
  • Sweat test
  • Nasal potential difference testing


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