Question

Formation of hypertonic urine is mediated through
A. Having a small loop of Henle
B. Eating a salt-free diet
C. Counter-current system
D. Increased water intake

Answer 1

Hint: Hypertonic means the urine has a lower water potential. This would also indicate an increase in the solute potential. So the urine is overall getting concentrated. This is set in place so that the average human does not produce a very large amount of diluted urine, resulting in huge water loss and consequently dehydration.

Complete answer:
The mechanism by which urine is getting concentrated (or becoming hypertonic) is called countercurrent multiplication.
Countercurrent multiplication happens with the help of a tubular structure, with an ascending and a descending limb. This is called the loop of Henle. The ascending limb of the loop of Henle is impermeable to water, but the descending limb is permeable to water.
The loop of Henle in juxtamedullary nephrons is large enough to force an osmotic gradient by the exchange of solutes and water.
The vasa recta capillaries run parallel to the loops of Henle and form an important part of this countercurrent system. The turn of the capillaries in a hairpin fashion slows the blood down. This helps in maintaining the osmotic gradient.
The steps are as follows:
The descending limb is permeable to water and solutes. And there is active reabsorption of solutes in the ascending limb to maintain the gradient in the interstitial area. So consequently, water moves out of the descending limb and solute moves in.
In the ascending limb, the thin part is permeable to the solute, but not to water. So water retains. In the thick part, the solute is actively reabsorbed, as mentioned above. This is a small dilution process. This results in overall hypertonic urine.

The formation of hypertonic urine is mediated through the counter-current system. So the correct option is ( C ).

Note:
Increased water intake and a salt-less diet will result in diluted urine, not concentrated one. And having a small loop of Henle does not aid in the concentration of urine any further. The formation of hypertonic urine is due to the proximity between the loop of henle and vasa recta, as well as the countercurrent in them.