GI System MCAT Practice Question
A 34-year-old woman with a 6-year history of ulcerative colitis presents with a 3-week history of worsening diarrhea (8-10 stools daily) despite optimization of her mesalamine therapy. Colonoscopy shows moderate inflammation with ulceration in the descending colon. Laboratory studies reveal normal pancreatic enzymes, normal bile acid levels, and normal fecal fat (ruling out malabsorption). Histopathology demonstrates disruption of tight junction proteins in the inflamed mucosa with preserved epithelial cell function. Which of the following best explains the mechanism of diarrhea in this patient?
Answer choices
- AIncreased secretion of chloride ions through CFTR channels on epithelial cells
- BParacellular movement of water and electrolytes from blood into the intestinal lumen due to disrupted claudins and occludinCorrect answer
- COsmotic effect from unabsorbed carbohydrates reaching the colon
- DImpaired transcellular sodium absorption via decreased SGLT1 and Na+/K+-ATPase expression
- EIncreased mucus production that sequesters intraluminal water
- FBacterial overgrowth producing short-chain fatty acids that increase colonic osmotic load
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