The presence of which facilitates the absorption of sodium in the lumen of the small intestine?

• A. Protein
• B. Potassium
• C. Glucose
• D. Vitamin D

Answer: (C)

Sodium absorption in the small intestine is driven by secondary active transport that uses the sodium gradient. The classic driver is the co-transport of sodium with glucose via the SGLT1 transporter on the brush-border membrane. When glucose is present in the lumen, Na+ and glucose are pulled into the enterocyte together. The driving Na+ gradient is maintained by the basolateral Na+/K+ ATPase, which pumps Na+ out of the cell, keeping intracellular Na+ low and allowing continued entry. Once inside, glucose exits to the bloodstream via GLUT transporters, while Na+ is pumped out again by Na+/K+ ATPase. So the presence of glucose directly facilitates Na+ absorption through this coupling mechanism. Other substances listed do not provide this Na+-coupled transport mechanism.