The correct answer is Option C: Both Statement I and Statement II are correct.

Here's why:

Statement I: Concentrated urine is formed due to the countercurrent mechanism in the nephron.

This statement is correct. The countercurrent mechanism in the nephron is crucial for forming concentrated urine. Here's how it works:

• Loop of Henle: The loop of Henle, a part of the nephron, creates a concentration gradient in the medullary interstitium. The descending limb of the loop is permeable to water but not to solutes. As filtrate flows down this limb, water moves out due to the increasing solute concentration in the interstitium, making the filtrate more concentrated. The ascending limb is impermeable to water but permeable to solutes, actively pumping out solutes like sodium, potassium, and chloride ions. This contributes to the higher solute concentration in the interstitium.
• Vasa Recta: The vasa recta, the capillaries surrounding the loop of Henle, also participate in maintaining the concentration gradient. They have a countercurrent flow, meaning blood flows in the opposite direction of the filtrate in the loop of Henle. This helps preserve the osmotic gradient by preventing rapid removal of solutes from the interstitium.
• Collecting Duct: As filtrate flows through the collecting duct, it encounters the concentrated medullary interstitium. Water moves out of the collecting duct, becoming more concentrated, ultimately leading to the production of concentrated urine.

Statement II: Countercurrent mechanism helps to maintain the osmotic gradient in the medullary interstitium.

This statement is also correct. The countercurrent mechanism is the primary mechanism responsible for establishing and maintaining the high osmotic gradient in the medullary interstitium. This gradient is crucial for the reabsorption of water and for the excretion of concentrated urine.

In summary, the countercurrent mechanism in the nephron is essential for producing concentrated urine and maintaining the osmotic gradient in the medullary interstitium. Therefore, both statements are accurate.