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Cystic Fibrosis 3

Cystic Fibrosis
physiology

Question

Vignette: A 25-year-old male presents to the emergency department with severe abdominal pain and vomiting. He has a history of cystic fibrosis. On physical examination, the patient appears dehydrated. His vital signs are as follows: heart rate is 110 beats per minute, blood pressure is 100/60 mmHg, respiratory rate is 22 breaths per minute, and temperature is 37.2°C (99°F). Blood tests reveal a high serum chloride concentration. Which of the following is the most likely mechanism responsible for this patient's findings?

Choices

A) Increased reabsorption of sodium in the distal tubule

B) Decreased reabsorption of chloride in the proximal tubule

C) Decreased secretion of potassium in the distal tubule

D) Decreased reabsorption of bicarbonate in the proximal tubule

E) Increased secretion of hydrogen ions in the proximal tubule

Answer

D) Decreased reabsorption of bicarbonate in the proximal tubule

Explanation

This patient's presentation of abdominal pain, vomiting, and history of cystic fibrosis suggests he is experiencing a metabolic alkalosis secondary to his disease. Cystic fibrosis is an autosomal recessive disorder characterized by defective chloride channels, resulting in the abnormal transport of chloride, sodium, and water across epithelial surfaces. This can lead to the dehydration seen in this patient. The high serum chloride concentration is due to a compensatory mechanism in response to the metabolic alkalosis. In metabolic alkalosis, there is an increase in bicarbonate concentration. To maintain electrical neutrality, there is a concurrent increase in the concentration of other anions such as chloride (Cl-) in the extracellular fluid. This patient's metabolic alkalosis is likely due to a decrease in reabsorption of bicarbonate in the proximal tubule, leading to a higher concentration of bicarbonate in the blood and thus a higher concentration of chloride as well.

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