A 50 years old male patient travels alone in a remote village of Rajasthan was not on any food or water for the past 24 hours. Urine osmolality is 1150 mOsm/Kg. The major site of water reabsorption is
**Core Concept**
The major site of water reabsorption in the kidneys is primarily determined by the concentration of the urine, which in turn is influenced by the body's hydration status and the presence of antidiuretic hormone (ADH). In states of dehydration, the kidneys increase water reabsorption to conserve body fluids.
**Why the Correct Answer is Right**
In this scenario, the patient has been without food or water for 24 hours, leading to dehydration. The urine osmolality of 1150 mOsm/Kg indicates that the kidneys are concentrating the urine to conserve water. The major site of water reabsorption is the collecting duct, where ADH stimulates the insertion of aquaporin-2 water channels into the apical membrane, increasing water permeability and reabsorption. This is a critical adaptation to conserve body fluids in states of dehydration.
**Why Each Wrong Option is Incorrect**
* **Option A:** The proximal convoluted tubule (PCT) is the primary site of glucose, amino acid, and electrolyte reabsorption, but it is not the major site of water reabsorption.
* **Option B:** The loop of Henle is responsible for creating the osmotic gradient in the medulla, but it is not the primary site of water reabsorption in the collecting duct.
* **Option D:** The distal convoluted tubule (DCT) and cortical collecting duct (CCD) are involved in fine-tuning electrolyte balance, but they are not the major site of water reabsorption in the collecting duct.
**Clinical Pearl / High-Yield Fact**
In states of dehydration, the kidneys increase water reabsorption in the collecting duct, but this adaptation can be impaired in conditions such as diabetes insipidus, where ADH deficiency leads to excessive urine production.
**Correct Answer:** C.