**Core Concept**
Glucose absorption in the small intestine occurs via secondary active transport, primarily through the SGLT1 transporter located in the apical membrane of enterocytes. This process relies on the electrochemical gradient of sodium ions (Na+) to drive glucose uptake from the lumen into the cell.

**Why the Correct Answer is Right**
The SGLT1 (sodium-glucose cotransporter 1) protein co-transports one glucose molecule with one sodium ion (Na+) into the enterocyte. The Na+ gradient, maintained by the Na+/K+ ATPase pump on the basolateral side, provides the driving force for glucose absorption. Without Na+, the transport mechanism cannot function efficiently, making Na+ essential for glucose uptake.

**Why Each Wrong Option is Incorrect**
Option B: K+ is not involved in glucose transport; it plays a role in membrane potential and fluid balance but not in glucose cotransport.
Option C: Ca2+ is involved in muscle contraction and bone metabolism, not in intestinal glucose absorption.
Option D: Cl− participates in fluid balance and acid-base regulation but does not directly facilitate glucose uptake.

**Clinical Pearl / High-Yield Fact**
Remember: Glucose absorption in the gut is **Na+-dependent** via SGLT1. This is why drugs like glyburide (a sulfonylurea) affect glucose metabolism and why sodium levels influence glucose absorption—key for understanding diabetes and gastrointestinal physiology.

✓ Correct Answer: A. Na+