Select the incorrect statement in following given options in relation to human kidney.

Options

A. Glomerular filtrate is isotonic in comparision to blood plasma
B. Due to maximum absorption in PCT filtrate becomes hypertonic
C. Active secretion of `H^+ & K^+` ions takes place in DCT
D. Some amount of urea passes from collecting duct to medullary interstitium and enters ascending limb of loop of Henle

Answer

To solve the question of identifying the incorrect statement regarding the human kidney, we can follow these steps:

### Step 1: Analyze the First Statement
**Statement:** Glomerular filtrate is isotonic in comparison to blood plasma.
**Evaluation:** This statement is correct. Isotonic means that the concentration of solutes in the glomerular filtrate is the same as that in blood plasma.

### Step 2: Analyze the Second Statement
**Statement:** Due to maximum absorption in PCT, filtrates become hypertonic.
**Evaluation:** This statement is also correct. In the proximal convoluted tubule (PCT), a significant amount of water and solutes are reabsorbed, making the filtrate hypertonic (higher solute concentration) compared to the original glomerular filtrate.

### Step 3: Analyze the Third Statement
**Statement:** Active secretion of H+ and K+ ions takes place in DCT.
**Evaluation:** This statement is incorrect. The secretion of H+ and K+ ions in the distal convoluted tubule (DCT) primarily occurs through passive processes, not active secretion. Therefore, this statement does not accurately describe the function of the DCT.

### Step 4: Analyze the Fourth Statement
**Statement:** Some amount of urea passes from the collecting duct to the medullary interstitium and enters the ascending limb of the loop of Henle.
**Evaluation:** This statement is correct. Urea does move from the collecting duct into the medullary interstitium and can re-enter the nephron at the ascending limb of the loop of Henle.

### Conclusion
After evaluating all the statements, we find that the third statement is the incorrect one.

**Final Answer:** The incorrect statement is: "Active secretion of H+ and K+ ions takes place in DCT."

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