**Core Concept**
The primary active step for sodium reabsorption in the proximal tubule is a critical process that involves the coordinated action of multiple transport proteins and mechanisms. This process helps maintain sodium balance and regulate blood pressure. The primary active step in sodium reabsorption is mediated by the sodium-potassium pump.

**Why the Correct Answer is Right**
The correct answer involves the sodium-potassium pump (Na+/K+-ATPase), a transmembrane enzyme that uses ATP to pump sodium ions out of the cell and potassium ions into the cell. This gradient is essential for the subsequent reabsorption of sodium ions in the proximal tubule. The sodium-potassium pump is located on the basolateral membrane of proximal tubule cells and plays a crucial role in establishing the electrochemical gradient that drives sodium reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically mention the sodium-potassium pump, which is the primary active step for sodium reabsorption in the proximal tubule.
**Option B:** This option is incorrect because it refers to the sodium-glucose cotransporter, which is a secondary active transport mechanism that reabsorbs glucose and sodium ions in the proximal tubule, but is not the primary active step.
**Option C:** This option is incorrect because it refers to the sodium-chloride cotransporter, which is primarily involved in sodium reabsorption in the thick ascending limb of the loop of Henle, not the proximal tubule.

**Clinical Pearl / High-Yield Fact**
The sodium-potassium pump plays a critical role in maintaining sodium balance and regulating blood pressure. Its dysfunction can lead to various electrolyte imbalances and cardiovascular diseases.

**Correct Answer:** C. The sodium-potassium pump (Na+/K+-ATPase) is the primary active step for sodium reabsorption in the proximal tubule.