Clinical Scenario:
A 2-month-old infant is admitted with recurrent dehydration and failure to thrive. The parents report profuse watery diarrhea since the neonatal period. On examination, the abdomen is distended but soft. There is no fever or blood in the stool.
Investigations show:
- Na⁺: 128 mmol/L
- K⁺: 2.6 mmol/L
- Cl⁻: 82 mmol/L
- HCO₃⁻: 37 mmol/L
- pH: 7.50
- Urine chloride: 8 mmol/L
- Stool chloride: 112 mmol/L
Which finding most strongly supports congenital chloride diarrhea rather than Bartter syndrome?
A. Hypokalemic metabolic alkalosis
B. Low serum chloride concentration
C. Low urine chloride concentration
D. High stool chloride concentration
E. Failure to thrive
Correct answer & Explanation:
Correct answer: D. High stool chloride concentration
Explanation
Both congenital chloride diarrhea and Bartter syndrome can produce hypokalemic, hypochloremic metabolic alkalosis, so these findings alone do not distinguish them.
In congenital chloride diarrhea, the primary problem is intestinal chloride loss due to defective SLC26A3 (Cl⁻/HCO₃⁻) exchange. Consequently, the stool contains a markedly elevated chloride concentration, typically >90 mmol/L.
The resulting volume and chloride depletion activates the renin–angiotensin–aldosterone system, causing renal potassium and hydrogen ion loss and producing metabolic alkalosis.
Urine chloride is usually low because the kidneys conserve chloride in response to extracellular volume depletion. In contrast, Bartter syndrome causes renal chloride wasting and therefore usually has an elevated urine chloride. However, stool chloride concentration is the more specific discriminator for congenital chloride diarrhea.
Why the other options are less useful
- A. Hypokalemic metabolic alkalosis: occurs in both conditions.
- B. Low serum chloride: occurs in both.
- C. Low urine chloride: supports extrarenal chloride loss, but the markedly high stool chloride is the defining clue.
- E. Failure to thrive: can occur in both severe chronic disorders.
Learning point:
Metabolic alkalosis + low urine Cl⁻ + very high stool Cl⁻ → congenital chloride diarrhea.
Metabolic alkalosis + high urine Cl⁻ → think renal chloride wasting, such as Bartter syndrome.
- Gils C, Eckhardt MC, Nielsen PE, Nybo M. Congenital chloride diarrhea: diagnosis by easy-accessible chloride measurement in feces. Case Rep Pediatr. 2016;2016:2519498.
- Höglund P, Hihnala S, Kujala M, et al. Mutations of the SLC26A3 (DRA) gene cause congenital chloride diarrhea. Nat Genet. 2001;27:208-211.
You can also practice another Scenario based MCQ on Constipation.
