Question

A 2-year-old child presents with watery diarrhea that subsides with fasting. Which of the following is most likely the cause?

• A. VIPoma
• B. Glucose-galactose malabsorption
• C. Cholera
• D. E. coli enterotoxin

Hint:

Diarrhea Differentiating Pearl:
$\bullet$ Osmotic Diarrhea (e.g., Lactase deficiency, Glucose-galactose malabsorption) $\rightarrow$ STOPS with Fasting | Stool Osmotic Gap $>100\text{ mOsm/kg}$ | $\text{Stool pH} < 5.5$.
$\bullet$ Secretory Diarrhea (e.g., Cholera, ETEC, VIPoma) $\rightarrow$ PERSISTS with Fasting | Stool Osmotic Gap $<50\text{ mOsm/kg}$.

Answer 1

The Correct Option is B

Concept:
Watery diarrhea is pathophysiologically classified into two major categories: Osmotic Diarrhea and Secretory Diarrhea.
A fasting trial (holding enteral oral intake and providing intravenous hydration) is the fundamental bedside diagnostic test used to differentiate between these two pathophysiological mechanisms.
Explanation:
• Osmotic Diarrhea: Results from the presence of unabsorbed, osmotically active solute molecules in the intestinal lumen that draw water passively across the mucosal barrier.

• A defining hallmark of osmotic diarrhea is that the diarrhea completely ceases or dramatically subsides when oral intake is discontinued (fasting). Furthermore, stool osmotic gap is elevated ($>100\text{ mOsm/kg}$).

• Glucose-galactose malabsorption is an autosomal recessive disorder caused by mutations in the SGLT1 cotransporter, leading to failure of monosaccharide absorption; unabsorbed glucose/galactose draws water into the lumen, causing severe osmotic diarrhea that completely stops during fasting or elimination of offending sugars.

• Secretory Diarrhea: Results from active mucosal secretion of chloride/fluid or inhibition of sodium absorption driven by enterotoxins or neuroendocrine hormones.

• Secretory diarrhea persists unabated even during periods of complete fasting starvation, with large stool volume ($>200\text{ mL/kg/day}$) and a low stool osmotic gap ($<50\text{ mOsm/kg}$).

• Cholera (Vibrio cholerae), enterotoxigenic E. coli (ETEC), and VIPomas (Vasoactive Intestinal Peptide secreting tumors) are classic prototypes of secretory diarrhea.
Final Answer:
Diarrhea that promptly subsides with fasting is osmotic in origin, which is characteristic of Glucose-galactose malabsorption.