Which of the following statements is INCORRECT?
I. Human kidneys can produce urine nearly six times more concentrated than the initial filtrate formed.
II. An increase in body fluid volume can switch on the osmoreceptors and stimulate ADH.
III. Substance like glucose, amino acids, `Na^+` , etc., in the filtrate, are reabsorbed actively by the renal tubule whereas the nitrogenous wastes are absorbed by passive transport
IV. Each kidney has nearly one million complex tubular structures called nephrons which are the functional units.

Options

A. I and II
B. III and IV
C. I, II and III
D. II, III and IV

Answer

To determine which statement is INCORRECT among the given options, let's analyze each statement one by one:

### Step-by-Step Solution:

1. **Statement I**: "Human kidneys can produce urine nearly six times more concentrated than the initial filtrate formed."
- **Analysis**: The human kidneys can concentrate urine, but they typically produce urine that is about four times more concentrated than the initial filtrate. Therefore, this statement is incorrect.

2. **Statement II**: "An increase in body fluid volume can switch on the osmoreceptors and stimulate ADH."
- **Analysis**: When body fluid volume increases, osmoreceptors actually get switched off, leading to a decrease in the secretion of Antidiuretic Hormone (ADH). Thus, this statement is also incorrect.

3. **Statement III**: "Substances like glucose, amino acids, Na+, etc., in the filtrate, are reabsorbed actively by the renal tubule whereas the nitrogenous wastes are absorbed by passive transport."
- **Analysis**: This statement is correct. Glucose, amino acids, and Na+ are reabsorbed actively, while nitrogenous wastes like urea are reabsorbed passively.

4. **Statement IV**: "Each kidney has nearly one million complex tubular structures called nephrons which are the functional units."
- **Analysis**: This statement is correct. Each kidney contains about one million nephrons, which are indeed the functional units of the kidney.

### Conclusion:
- The incorrect statements are **I** and **II**. However, since the question asks for which statement is INCORRECT, we can conclude that **Statement I** is the primary incorrect statement regarding the concentration of urine.

### Final Answer:
**Statement I is INCORRECT.**

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