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:
Age: 6–24 months (most common)
Profuse watery diarrhea
Vomiting prominent
No blood or fecal leukocytes
Metabolic acidosis from bicarbonate loss
Hypernatremic dehydration possible
Pathophysiology (PG concept):
Rotavirus produces NSP4, a viral enterotoxin that:Stimulates chloride secretion
Activates the enteric nervous system
Causes secretory diarrhea
Also leads to villous blunting → secondary lactose malabsorption (osmotic component)
Thus, diarrhea is primarily secretory, with a secondary osmotic component.
Why others are incorrect:
Direct invasion → seen in Shigella, Salmonella
Adenylate cyclase activation → typical of cholera
Pure osmotic mechanism → not the primary mechanism
Cytokine-mediated hypermotility → not the main mechanism
