Clinical meaning
Diarrhea is defined as the passage of three or more loose or watery stools per day, or more frequent passage than is normal for the individual. It is classified by duration: acute (less than 14 days), persistent (14-30 days), and chronic (greater than 30 days). Understanding the four pathophysiologic mechanisms is essential for targeted management. Osmotic diarrhea occurs when poorly absorbed solutes create an osmotic gradient that draws water into the intestinal lumen. Classic causes include lactose intolerance (undigested lactose draws water), magnesium-containing laxatives or antacids, sorbitol or mannitol (sugar-free products), and malabsorption syndromes. A key feature is that osmotic diarrhea stops with fasting or removal of the offending agent. Secretory diarrhea results from active secretion of electrolytes and water into the intestinal lumen exceeding absorptive capacity. This is mediated by enterotoxins (cholera toxin, E. coli heat-labile toxin) that activate adenylate cyclase or guanylate cyclase, increasing intracellular cAMP or cGMP and driving chloride and water secretion. Secretory diarrhea characteristically persists during fasting and produces large-volume watery stool without blood or white blood cells. Inflammatory (exudative) diarrhea occurs when mucosal inflammation...