## **Core Concept**
The renal handling of water is primarily regulated by vasopressin (also known as antidiuretic hormone, ADH), which plays a crucial role in water reabsorption in the kidneys. In the absence of vasopressin, the body loses its ability to properly concentrate urine, leading to an increase in urine output.

## **Why the Correct Answer is Right**
The correct answer, , indicates that in the absence of vasopressin, the greatest fraction of filtered water is absorbed in the proximal convoluted tubule (PCT). The PCT is responsible for the reabsorption of approximately 65-70% of the filtered water and electrolytes back into the bloodstream. This process is isotonic and does not require vasopressin. The reabsorption in the PCT is driven by the reabsorption of solutes like sodium, glucose, and amino acids, which creates an osmotic gradient favoring water reabsorption.

## **Why Each Wrong Option is Incorrect**
* **Option A:** The loop of Henle does reabsorb water, but the amount is significantly less compared to the PCT, and its function is more related to creating a concentration gradient for water reabsorption in the collecting duct, which is regulated by vasopressin.
* **Option B:** The distal convoluted tubule (DCT) does reabsorb some water, but again, the fraction is much smaller than in the PCT, and its reabsorption is also influenced by aldosterone and other factors rather than vasopressin directly.
* **Option D:** The collecting duct's ability to reabsorb water is highly dependent on vasopressin. In the absence of vasopressin, almost no water is reabsorbed in the collecting duct, making it an incorrect answer for where the greatest fraction of filtered water is absorbed.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the action of vasopressin (ADH) primarily affects the collecting ducts, increasing their permeability to water and thus allowing more water to be reabsorbed back into the bloodstream, which concentrates the urine. In diabetes insipidus, a condition characterized by the deficiency of vasopressin or its action, patients produce large volumes of dilute urine.

## **Correct Answer: .**