## **Core Concept**
The nephron is the functional unit of the kidney responsible for filtering waste and excess substances from the blood. It consists of several parts, including the glomerulus, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct. The permeability to water varies along different segments of the nephron.

## **Why the Correct Answer is Right**
The **descending limb of the loop of Henle** is highly permeable to water due to the presence of aquaporin-1 water channels. In contrast, the **ascending limb of the loop of Henle**, particularly the **thick ascending limb**, is impermeable to water. This segment is crucial for diluting the filtrate and maintaining the osmotic gradient in the medulla. The impermeability to water in this part is due to the absence of aquaporin-1 water channels.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **proximal convoluted tubule (PCT)** is highly permeable to water, reabsorbing approximately 65-70% of the filtered water back into the bloodstream.
- **Option B:** The **descending limb of the loop of Henle** is permeable to water, allowing water to be reabsorbed into the hyperosmotic medullary interstitium.
- **Option D:** The **collecting duct** has regulated permeability to water, controlled by antidiuretic hormone (ADH), which inserts aquaporin-2 water channels into the apical membrane, making it permeable to water.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that the loop diuretics, such as furosemide, act on the **thick ascending limb of the loop of Henle**. This site of action is critical for their potent diuretic effect, as this segment is impermeable to water but actively reabsorbs ions (sodium, potassium, and chloride), leading to increased urine output.

## **Correct Answer:** . Thick ascending limb of loop of Henle