GI Nutrition, Absorption, and Motility 1 — Questions and Answers
Question 1: Fat-soluble vitamins (A, D, E, K) are primarily absorbed via which mechanism in the small intestine?
- Active transport via SGLT1
- Incorporation into micelles and passive diffusion into enterocytes (Correct answer)
- Facilitated diffusion via GLUT5
- Pinocytosis via intestinal macrophages
Correct answer: Incorporation into micelles and passive diffusion into enterocytes
Fat-soluble vitamins are solubilized in bile acid micelles, then passively diffuse across the enterocyte brush border and are packaged into chylomicrons.
Question 2: Vitamin B12 absorption requires intrinsic factor and is completed in which GI segment?
- Duodenum
- Proximal jejunum
- Terminal ileum (Correct answer)
- Cecum
Correct answer: Terminal ileum
The B12–intrinsic factor complex binds to cubilin receptors exclusively in the terminal ileum for absorption by receptor-mediated endocytosis.
Question 3: Which electrolyte is primarily absorbed in the colon via aldosterone-regulated sodium channels (ENaC)?
- Potassium
- Calcium
- Sodium (Correct answer)
- Magnesium
Correct answer: Sodium
Aldosterone upregulates epithelial sodium channels (ENaC) and Na+/K+-ATPase in colonocytes, driving sodium reabsorption and potassium secretion.
Question 4: Short bowel syndrome most commonly causes diarrhea and malabsorption when the remaining small bowel is less than how many centimeters?
- 300 cm
- 200 cm
- 100 cm (Correct answer)
- 50 cm
Correct answer: 100 cm
Short bowel syndrome typically occurs when less than 100–150 cm of functional small intestine remains after resection, insufficient for adequate nutrient absorption.
Question 5: Chylomicrons transport dietary lipids from the enterocyte to systemic circulation via which route?
- Portal vein directly to the liver
- Intestinal lacteals → mesenteric lymphatics → thoracic duct → systemic circulation (Correct answer)
- Direct absorption into capillaries and portal vein
- Bile acid recycling via enterohepatic circulation
Correct answer: Intestinal lacteals → mesenteric lymphatics → thoracic duct → systemic circulation
Chylomicrons enter lacteals (intestinal lymphatics), travel through mesenteric lymphatics and the thoracic duct, and enter the bloodstream at the left subclavian vein.
Question 6: Iron absorption is primarily regulated at which level and is enhanced by which concurrent intake?
- Renal reabsorption; calcium
- Intestinal enterocyte uptake; vitamin C (ascorbic acid) (Correct answer)
- Hepatic storage; vitamin B12
- Bone marrow utilization; vitamin D
Correct answer: Intestinal enterocyte uptake; vitamin C (ascorbic acid)
Iron absorption is regulated by duodenal enterocytes and is enhanced by vitamin C, which keeps iron in the soluble ferrous (Fe2+) form for transport by DMT1.
GI Nutrition, Absorption, and Motility 1
Fat-soluble vitamins (A, D, E, K) are primarily absorbed via which mechanism in the small intestine?