**Core Concept**
Glucose symport is a secondary active transport mechanism where glucose is co-transported into cells along with another ion, typically sodium (Na⁺), using the electrochemical gradient of Na⁺. This process occurs primarily in the proximal convoluted tubule of the kidney, where glucose is reabsorbed from the filtrate back into the bloodstream.

**Why the Correct Answer is Right**
The sodium-glucose cotransporter (SGLT2) in the renal tubule utilizes the inward sodium gradient (maintained by the Na⁺/K⁺-ATPase pump) to drive glucose reabsorption. Glucose moves into the tubular cell with Na⁺ via symport, not independently. This process is critical for maintaining blood glucose levels and preventing glycosuria. The Na⁺ gradient is the primary driving force, making Na⁺ essential for glucose symport.

**Why Each Wrong Option is Incorrect**
Option B: Ca++ is not involved in glucose transport; it plays roles in muscle contraction and bone metabolism, not renal glucose reabsorption.
Option C: K+ does not participate in glucose symport; it is mainly involved in membrane potential and cellular function.
Option D: Cl- is a key player in osmotic balance and neuronal function but is not directly involved in glucose transport via symport.

**Clinical Pearl / High-Yield Fact**
In diabetes mellitus, when blood glucose exceeds the renal threshold (~180 mg/dL), glucose spills into urine (glycosuria) because the SGLT2 transporters become saturated. This is why SGLT2 inhibitors are used in type 2 diabetes to promote glucose excretion.

✓ Correct Answer: A. Na+