Brush border oligosaccharidase deficiency leads to diarrhea and bloating after ingestion of sugar due to which primary mechanism?

• A. Water shifts into the intestinal lumen due to osmotic activity of undigested oligosaccharides
• B. Increased water absorption into the bloodstream
• C. Reduced luminal pressure from dehydration
• D. Decreased bacterial fermentation and gas production

Answer: (A)

When brush border oligosaccharidases are deficient, undigested oligosaccharides remain in the intestinal lumen. These carbohydrates are osmotically active, so they draw water into the gut. That osmotic influx increases stool water content, producing diarrhea after sugar ingestion. The same undigested sugars are also fermented by colonic bacteria, which generates gas and leads to bloating. The primary driver of the diarrhea is the osmotic water shift from the presence of undigested sugars; options proposing increased water absorption, reduced luminal pressure, or decreased fermentation don’t fit the main mechanism.