“Diluting segment” of a nephron is:
**Core Concept**
The nephron's diluting segment plays a crucial role in regulating the amount of water and solutes reabsorbed in the kidneys. This segment is characterized by its ability to dilute or concentrate the urine, depending on the body's needs.
**Why the Correct Answer is Right**
The diluting segment of the nephron is located in the early distal convoluted tubule and is responsible for reabsorbing sodium and water. This process is mediated by the Na-K-2Cl cotransporter (NKCC2), a key enzyme involved in sodium reabsorption. The diluting segment also expresses aquaporin-1 (AQP1), a water channel that facilitates water reabsorption. As a result, the urine becomes more dilute, helping to conserve water and electrolytes.
**Why Each Wrong Option is Incorrect**
**Option A:** The proximal convoluted tubule (PCT) is primarily responsible for bulk reabsorption of glucose, amino acids, and other nutrients, but it is not the diluting segment.
**Option B:** The loop of Henle, particularly its thick ascending limb, is involved in salt reabsorption and is not the diluting segment.
**Option C:** The collecting duct is responsible for the final regulation of water and electrolytes, but it is not the primary diluting segment.
**Clinical Pearl / High-Yield Fact**
The diluting segment's ability to concentrate or dilute the urine is essential for maintaining proper fluid balance and electrolyte homeostasis in the body.
**Correct Answer: C. Collecting duct.**