This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
The Principal site of absorption of sodium is 
Select and explain. 
A. Proximal convoluted tubule, B. Distal convoluted tubule, C. Loop of Henle (thick portion), D. Collecting duct
A: Proximal convoluted tubule
Answer: A: Proximal convoluted tubule 
Explanation: Other than at thin descending limb of the loop of Henle, all other sites of tubule Na+ are reabsorbed by various mechanisms. 
-        Proximal tubule- 60-70%

Na+- H+ exchange
Na+- Glucose co-transport
Na+- amino acid co-transport
Na+ unitransport

-        The thick ascending limb of the loop of Henle - 30%

Na+- 2Cl-- K+ co-transport

-        Early distal tubule- 7%

Na+- Cl- co-transport

-        Late distal tubule and collecting duct - 3%

ENaC channels