The Question is from neet-zoology excretory-products-and-their-elimination , and it was created by the author A SUBRAMANYAM

Question

A rare genetic disorder leads to impaired urea transport across cell membranes within the nephron. Which region's function would be MOST directly compromised, leading to a significant disruption in the establishment of the medullary concentration gradient?

A SUBRAMANYAM · Accepted Answer

## CORE INFORMATION
✓ **Answer**: D
## CONCEPT FRAMEWORK
	**Primary Concept**: Medullary Concentration Gradient and Urea Recycling
	**Related Topics**: Nephron function, Osmoregulation, Water balance, Urea handling
	**Prerequisites**: Understanding of the nephron segments and their roles in urine formation.
## SOLUTION PATHWAY
**Given Information**:
	Impaired urea transport across cell membranes within the nephron.
**Method & Steps**:
	1. **Identify the role of urea in the medullary concentration gradient**: Urea contributes significantly to the high osmolarity of the medulla, which is essential for concentrating urine.
		Reasoning:  The medullary concentration gradient allows water to be reabsorbed from the collecting duct under the influence of ADH (antidiuretic hormone).
	2. **Determine where urea is primarily handled in the nephron**: The collecting duct is the primary site of regulated urea reabsorption into the medullary interstitium.
		Working: Urea transporters are present in the collecting duct cells, allowing urea to move passively down its concentration gradient, which is established by ADH.
		Key Point: While urea is filtered in the glomerulus and reabsorbed to some extent in other segments, the collecting duct plays the most crucial role in regulating its concentration in the medulla.
	3. **Relate impaired urea transport to the affected region**: If urea transport is impaired, the collecting duct will not be able to effectively reabsorb urea into the medulla.
		Result: This will disrupt the medullary concentration gradient, impairing the kidney's ability to concentrate urine.
		Verification: The collecting duct's role in urea recycling is central to establishing the osmotic gradient necessary for water reabsorption.
## OPTION ANALYSIS
[A] :x: Proximal Convoluted Tubule: While the proximal tubule reabsorbs a significant amount of filtered urea, it doesn't directly contribute to the medullary concentration gradient in the same way as the collecting duct. Impairment here would have less of a direct effect on the gradient.
[B] :x: Distal Convoluted Tubule: The distal convoluted tubule is involved in fine-tuning electrolyte balance and not primarily involved in urea handling. Its function is less directly related to the medullary concentration gradient.
[C] :x: Thick Ascending Limb of the Loop of Henle: The thick ascending limb is important for creating the cortico-medullary osmotic gradient by actively transporting $Na^+$ , $K^+$ , and $Cl^-$  ions.  It doesn't directly handle urea.
[D] ✓: Collecting Duct:  The collecting duct is where regulated urea reabsorption occurs, contributing significantly to the high osmolarity of the medulla. Impaired urea transport here would directly compromise the medullary concentration gradient.
## LEARNING AIDS
	**Common Mistakes**:
		1. Focusing solely on where urea is filtered or initially reabsorbed, rather than its role in the medullary concentration gradient.
			How to avoid: Remember that the key is the *medullary concentration gradient*, which is most directly impacted by urea handling in the collecting duct.
		2. Confusing the roles of the different nephron segments.
			How to avoid: Review the specific functions of each segment and their contributions to urine formation.
	**Quick Tips**:
		1. ADH enhances urea permeability in the collecting duct, linking water and urea reabsorption.
		2. The vasa recta (blood vessels in the medulla) help maintain the medullary concentration gradient by preventing the washout of urea and other solutes.

A SUBRAMANYAM · Answer

Proximal Convoluted Tubule

A SUBRAMANYAM · Answer

Distal Convoluted Tubule

A SUBRAMANYAM · Answer

Thick Ascending Limb of the Loop of Henle

A SUBRAMANYAM · Answer

The correct Option is "Collecting Duct"

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