This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
PCT absorbs all except: 
Select and explain. 
A. Sodium, B. Glucose, C. Hydrogen ions, D. Amino acids
C: Hydrogen ions
Answer: C: Hydrogen ions 
Explanation: PCT reabsorbs all of the filtered glucose and amino acids. PCT reabsorbs largest fraction (about 67%) of the filtered Na+, K+, Ca2+, Cl-, HCO3-, and water H+ is secreted by the PCT; it is for the re-absorption of the filtered HCO3-. Fluid at the end of the PCT is always Iso-osmotic to plasma. Glomerulotubular balance in the proximal tubule:- This phenomenon tends to minimize the effects of a fluctuating GFR on Na+ and water excretion, which promotes a constant plasma volume. It maintains Constant fractional reabsorption (about 67%) of the Filtered Na+ and water, even if the GFR varies.