A 1-month-old boy is brought to the office on a hot summer day due to vomiting and excessive sleepiness. He had been exclusively breastfeeding with normal feeding patterns until this afternoon, when he had 2 episodes of emesis and appeared much sleepier than usual. The infant was born at 41 weeks gestation after an unremarkable pregnancy. Newborn screening was positive for a homozygous ΔF508 mutation affecting an epithelial transmembrane protein; further confirmatory testing is pending. Physical examination shows a somnolent boy with a sunken fontanelle and dry mucous membranes. Which of the following electrolyte disturbances is most likely responsible for this patient's current symptoms?

Question

A 1-month-old boy is brought to the office on a hot summer day due to vomiting and excessive sleepiness.  He had been exclusively breastfeeding with normal feeding patterns until this afternoon, when he had 2 episodes of emesis and appeared much sleepier than usual.  The infant was born at 41 weeks gestation after an unremarkable pregnancy.  Newborn screening was positive for a homozygous ΔF508 mutation affecting an epithelial transmembrane protein; further confirmatory testing is pending.  Physical examination shows a somnolent boy with a sunken fontanelle and dry mucous membranes.  Which of the following electrolyte disturbances is most likely responsible for this patient's current symptoms?

Quizplus · Accepted Answer

11ec160b_3b36_2ccb_9249_1d37ac176dbd_MD0001_00 The ΔF508 mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) protein is the most common mutation in patients with cystic fibrosis (CF). In unaffected individuals, the CFTR protein serves as a chloride channel that regulates the flow of sodium, chloride, and water across the epithelial membranes of the airways, biliary tree, intestines, sweat ducts, and pancreatic ducts. Newborn screening for CF is performed in the United States, but positive results should be confirmed by sweat testing. When eccrine sweat is first produced, it is isotonic with extracellular fluid. As sweat travels through the eccrine duct to the skin's surface, chloride is normally resorbed via CFTR, and sodium and water follow. CFTR facilitates production of hypotonic sweat in patients without CF. However, patients with CF are unable to reabsorb chloride and sodium in the eccrine ducts and therefore secrete sweat with high sodium and chloride levels. The most likely cause of this patient's symptoms (lethargy, vomiting) is hyponatremia due to excessive salt wasting from his sweat. Risk factors for hyponatremia in patients with CF include exclusive breast or formula feeding prior to the introduction of sodium-rich, solid foods; exposure to high-temperature environments; and exercise. Therefore, salt supplementation is recommended for patients with CF. (Choice A) Chloride ions are an important contributor to extracellular fluid volume, and hypochloremia is also very common in patients with CF due to sweat losses. In contrast, hyperchloremia is more likely to develop in normal patients with excessive sweating due to increased free water loss from hypotonic sweat. (Choice B) Hyperkalemia typically occurs with cell lysis (eg, rhabdomyolysis, tumor lysis syndrome), metabolic acidosis (due to intracellular to extracellular K⁺ shift), or abnormalities with renal excretion. (Choice C) Hypernatremia classically occurs with excessive salt intake or free water loss (eg, diarrhea, diabetes insipidus, improperly prepared infant formula). Patients with CF typically do not experience hypernatremia as they lose excess sodium through their sweat glands. (Choice D) Hypomagnesemia most commonly arises from urinary loss due to medications (eg, loop diuretics), alcohol use, or uncontrolled diabetes mellitus with polyuria. Educational objective: Patients with cystic fibrosis (CF) produce eccrine sweat with higher-than-normal concentrations of sodium and chloride. Exposure to high temperature or exercise can lead to hyponatremia and hypochloremia due to excessive sodium chloride loss through sweat; therefore, salt supplementation is recommended. __________ References: Hyponatremic dehydration as a presentation of cystic fibrosis. (http://www.ncbi.nlm.nih.gov/pubmed/17110865) Prevalence of hyponatremia at diagnosis and factors associated with the longitudinal variation in serum sodium levels in infants with cystic fibrosis. (http://www.ncbi.nlm.nih.gov/pubmed/22404954) Electrolyte abnormalities in cystic fibrosis: systematic review of the literature. (http://www.ncbi.nlm.nih.gov/pubmed/24326787)

A 1-Month-Old Boy Is Brought to the Office on a Hot