Antidiuretic hormone affects the kidney by :

Options

A. constricting arterioles and thus raising blood pressure
B. increasing the reabsorption of `Na^(+)` in the distal convoluted tubules
C. countering the renin-angiotensin-aldosterone mechanism
D. increasing the water permeability of the distal convoluted tubule and collecting duct, thus increasing the osmolarity of the urine

Explanation

To answer the question regarding how antidiuretic hormone (ADH) affects the kidney, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding ADH**: Antidiuretic hormone, also known as vasopressin, is a hormone produced by the hypothalamus and released from the posterior pituitary gland. Its primary role is to regulate water balance in the body.

2. **Function of ADH in the Kidneys**: ADH primarily acts on the kidneys, specifically on the distal convoluted tubule and the collecting ducts. Its main function is to increase the reabsorption of water from the urine back into the bloodstream.

3. **Mechanism of Action**: When ADH is present, it binds to receptors on the cells of the distal convoluted tubule and collecting ducts. This binding triggers the insertion of aquaporin channels (water channels) into the cell membranes, which increases the permeability of these tubules to water.

4. **Effect on Urine Concentration**: As a result of increased water reabsorption, less water is excreted in the urine. This leads to a more concentrated urine (higher osmolarity) because the amount of solute remains relatively constant while the volume of water decreases.

5. **Conclusion**: Therefore, the correct answer to the question is that ADH increases the water permeability of the distal convoluted tubule and collecting ducts, which in turn increases the osmolarity of urine.

### Final Answer:
Antidiuretic hormone affects the kidney by increasing the water permeability of the distal convoluted tubule and collecting ducts, thus increasing the osmolarity of urine.

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