Which of the following statements is/are incorrect regarding the collecting duct?
(i). It extends from the cortex to medulla.
(ii). Large amount of water could be reabsorbed by it to produce concentrated urine.
(iii). Small amount of urea diffuses into it from the medulla to keep up the osmolarity.
(iv). it plays a role in maintaining pH and ionic balance of blood by the selective secretion of `H^(+)` and `K^(+)` ions.

Options

A. Only (i)
B. Only (iii)
C. (ii) and (iii)
D. (i) and (iv)

Explanation

To determine which statements regarding the collecting duct are incorrect, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Statement (i): It extends from the cortex to medulla.**
- The collecting duct indeed extends from the renal cortex (outer part of the kidney) down into the renal medulla (inner part of the kidney). This statement is **true**.

2. **Statement (ii): Large amount of water could be reabsorbed by it to produce concentrated urine.**
- The collecting duct is responsible for the reabsorption of water, particularly under the influence of the hormone vasopressin (ADH). This allows for the production of concentrated urine. Therefore, this statement is also **true**.

3. **Statement (iii): Small amount of urea diffuses into it from the medulla to keep up the osmolarity.**
- This statement is **incorrect**. In reality, small amounts of urea diffuse from the collecting duct into the medulla to help maintain osmolarity in the medullary interstitium, not the other way around.

4. **Statement (iv): It plays a role in maintaining pH and ionic balance of blood by the selective secretion of H⁺ and K⁺ ions.**
- The collecting duct does indeed help maintain the pH and ionic balance of the blood by selectively secreting hydrogen ions (H⁺) and potassium ions (K⁺). This statement is **true**.

### Conclusion:
The only incorrect statement regarding the collecting duct is statement (iii).

### Final Answer:
The incorrect statement is **(iii)**.

---