## **Core Concept**
The question tests understanding of renal physiology, specifically how vasopressin (also known as antidiuretic hormone, ADH) influences water reabsorption in the kidneys. Vasopressin plays a crucial role in regulating water balance in the body by affecting the permeability of renal tubules to water.

## **Why the Correct Answer is Right**
In the presence of vasopressin, the greatest fraction of filtered water is absorbed in the collecting ducts. Vasopressin increases the permeability of the collecting ducts to water by stimulating the insertion of **aquaporin-2** water channels into the apical membrane of collecting duct cells. This allows for the reabsorption of water back into the interstitium, concentrating the urine and reducing water loss. The collecting ducts are key sites for fine-tuning water reabsorption under the influence of vasopressin.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The proximal convoluted tubule (PCT) reabsorbs a significant fraction of filtered water (about 65-70%) passively, following the reabsorption of solutes like sodium and glucose, but this process is not primarily influenced by vasopressin.
- **Option B:** The loop of Henle, particularly the thick ascending limb, is impermeable to water but reabsorbs ions. The descending limb of the loop of Henle is permeable to water, but again, vasopressin's main effect is not here.
- **Option C:** This option might seem plausible but is not correct based on the primary site of vasopressin action.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the action of vasopressin (ADH) can be mimicked by **desmopressin**, a synthetic analogue used clinically to manage conditions like diabetes insipidus. Remembering that vasopressin's primary role in water reabsorption occurs in the collecting ducts helps in understanding the mechanism of action of such drugs.

## **Correct Answer: .**