In the presence of vasopressin the greatest fraction of filtered water is re-absorbed which pa of the nephron:
**Core Concept**
The nephron's ability to reabsorb water is tightly regulated by vasopressin (antidiuretic hormone, ADH), which increases water permeability in the collecting ducts. This process is essential for maintaining proper water balance in the body.
**Why the Correct Answer is Right**
Vasopressin acts on the V2 receptors in the collecting duct cells, leading to an increase in the expression and insertion of aquaporin-2 (AQP2) water channels into the apical membrane. This results in a significant increase in water permeability, allowing the collecting ducts to reabsorb a large fraction of the filtered water. The collecting ducts, particularly the cortical and medullary collecting ducts, play a crucial role in this process.
**Why Each Wrong Option is Incorrect**
* **Option A:** The proximal convoluted tubule (PCT) reabsorbs a significant amount of water, but it does not respond to vasopressin. The PCT's water reabsorption is largely passive, driven by the concentration gradient.
* **Option B:** The loop of Henle reabsorbs water, but its response to vasopressin is minimal compared to the collecting ducts. The thick ascending limb of the loop of Henle is impermeable to water.
* **Option C:** The distal convoluted tubule (DCT) reabsorbs some water, but it is not the primary site of water reabsorption in response to vasopressin. The DCT's water reabsorption is also largely passive.
**Clinical Pearl / High-Yield Fact**
The collecting duct's response to vasopressin is a critical mechanism for regulating water balance in the body. A deficiency in vasopressin or its receptors can lead to diabetes insipidus, characterized by excessive water loss and polyuria.
**Correct Answer:** C. Collecting ducts. Collecting ducts.