Regulation Of Kidney Function

Step 1: Understand nephron function.
The nephron is the functional unit of the kidney, responsible for filtering blood and producing urine.
Step 2: Identify the site of sodium reabsorption.
The proximal convoluted tubule is the site where the majority of sodium reabsorption occurs, via active transport mechanisms.
The renal capsule is responsible for filtering blood but not for reabsorption. The collecting duct and the descending arm of Henle’s loop play roles in water reabsorption but are not the primary sites for active sodium reabsorption.
Conclusion: The correct answer is (A) Proximal convoluted tubule.