Counter-current mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from the

Options

A. Vasa recta
B. Henle’s loop
C. Collecting tubuleÂ
D. DCTÂ

Explanation

**Step-by-Step Solution:**

1. **Understanding the Counter-current Mechanism:**
- The counter-current mechanism is a process that helps maintain a concentration gradient in the kidney, specifically in the medullary interstitium. This mechanism involves the flow of fluids in opposite directions, which enhances the efficiency of reabsorption and secretion processes in the nephron.

2. **Identifying the Regions of the Kidney:**
- The kidney consists of two main regions: the cortex (outer region) and the medulla (inner region). The medulla contains structures such as the loop of Henle and the collecting ducts, which are crucial for urine concentration.

3. **Role of the Medullary Interstitium:**
- The medullary interstitium is the surrounding area of the nephron in the medulla. It plays a significant role in the reabsorption of water and solutes from the filtrate back into the bloodstream.

4. **Function of the Loop of Henle:**
- The loop of Henle has two parts: the descending limb, which is permeable to water, allowing water to be reabsorbed into the interstitium, and the ascending limb, which is impermeable to water but allows for the reabsorption of salts. This creates a concentration gradient in the medullary interstitium.

5. **Function of the Collecting Duct:**
- The collecting duct is involved in the final concentration of urine. It is primarily impermeable to water unless influenced by the antidiuretic hormone (ADH). When ADH is present, it allows for the reabsorption of water from the collecting duct into the interstitium, further concentrating the urine.

6. **Conclusion:**
- The presence of a concentration gradient in the medullary interstitium facilitates the easy passage of water from the nephron into the interstitium, primarily occurring in the descending limb of the loop of Henle and the collecting duct when influenced by ADH.

7. **Final Answer:**
- The presence of such an interstitial gradient helps in the easy passage of water from the **collecting duct**.

---