Clinical Scenario:
An 18-month-old previously healthy boy is brought to the emergency department with 3 days of profuse watery diarrhea and multiple episodes of non-bilious vomiting. He has low-grade fever and decreased oral intake. There is no blood or mucus in stools. On examination, he is irritable, has sunken eyes, dry mucous membranes, delayed capillary refill (3 seconds), and tachycardia.
Laboratory findings:
Serum sodium: 150 mEq/L
Serum potassium: 3.4 mEq/L
Serum bicarbonate: 16 mEq/L
Stool microscopy shows no leukocytes or red blood cells.
Which of the following best explains the primary mechanism of diarrhea in this patient?
A. Direct enterocyte invasion causing mucosal ulceration
B. Enterotoxin-mediated activation of adenylate cyclase
C. Osmotic diarrhea due to lactase deficiency secondary to villous injury
D. Secretory diarrhea due to activation of enteric nervous system by viral NSP4 protein
E. Systemic cytokine-mediated intestinal hypermotility
Correct answer & Explanation:
Correct Answer: D. Secretory diarrhea due to activation of enteric nervous system by viral NSP4 protein
Discussion: Rotavirus Gastroenteritis
This presentation is typical of rotavirus gastroenteritis, an important cause of acute gastroenteritis in infants and young children. Rotavirus commonly causes vomiting followed by profuse watery diarrhea, fever, and dehydration. The first infection is usually the most severe.
The diarrhea is non-bloody and generally non-inflammatory. Severe disease may result in dehydration, electrolyte abnormalities, and metabolic acidosis.
Mechanism of diarrhea
Rotavirus diarrhea has more than one mechanism, but a major mechanism is secretory diarrhea related to the viral protein NSP4.
NSP4 acts as a viral enterotoxin and increases intracellular calcium in intestinal epithelial cells. It can stimulate enterochromaffin cells to release serotonin, which activates the enteric nervous system and promotes intestinal secretion and motility. This contributes to watery diarrhea.
Rotavirus also damages mature villous enterocytes and reduces absorptive function. Brush-border enzymes, including lactase, may be reduced, producing a secondary malabsorptive/osmotic component. Therefore, rotavirus diarrhea is best regarded as having both secretory and malabsorptive components rather than being purely secretory.
Clinical features important for pediatric exams
• Young infant or child, particularly during the first few years of life
• Vomiting is common and may precede diarrhea
• Profuse watery, non-bloody diarrhea
• Fever may occur
• Dehydration is the major clinical complication
• Severe cases may develop electrolyte abnormalities and metabolic acidosis
• Illness is usually self-limited over several days
• Management is primarily supportive, with oral rehydration as the mainstay of treatment
Why the other options are incorrect
Direct intestinal invasion is more characteristic of invasive bacterial enteritis, such as Shigella, which produces inflammatory diarrhea and may cause blood and fecal leukocytes.
Adenylate cyclase activation with increased intracellular cAMP is classically associated with cholera toxin and results in marked intestinal chloride and water secretion.
Pure osmotic diarrhea is not the main mechanism in rotavirus. Although villous injury and reduced brush-border enzymes can cause malabsorption and contribute to osmotic diarrhea, secretory mechanisms also play an important role.
Cytokine-mediated hypermotility is not the principal mechanism responsible for the characteristic watery diarrhea of rotavirus.
Board Review Pearl
In a young child with vomiting followed by profuse watery diarrhea, think of rotavirus. If the question asks specifically about the mechanism of diarrhea, remember:
Rotavirus → NSP4 → enteric nervous system activation → intestinal secretion and increased motility → watery diarrhea
The overall mechanism is multifactorial, with an important secretory component and a secondary malabsorptive component.
References
- Crawford SE, Ramani S, Tate JE, Parashar UD, Svensson L, Hagbom M, et al. Rotavirus infection. Nat Rev Dis Primers. 2017;3:17083.
- Dennehy PH. Rotavirus infection: an update on management and prevention. Adv Pediatr. 2012;59(1):245-255.
- CDC. Rotavirus. In: Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book). Centers for Disease Control and Prevention.
You can also practice more high yield clinical scenario based MCQs on Gastroenterology.
