The Question is from neet-zoology excretory-products-and-their-elimination function-of-the-tubules, and it was created by the author Mr. BALACHANDRA.N

Question

A patient presents with metabolic acidosis and significantly reduced bicarbonate reabsorption in their proximal convoluted tubule. Which of the following transporters is MOST likely malfunctioning?

Mr. BALACHANDRA.N · Accepted Answer

## CORE INFORMATION
✓ **Answer**: A
## CONCEPT FRAMEWORK
 **Primary Concept**: Renal bicarbonate reabsorption
 **Related Topics**: Metabolic acidosis, proximal convoluted tubule transporters
 **Prerequisites**: Understanding of kidney physiology, particularly the mechanisms of bicarbonate reabsorption in the proximal tubule.
## SOLUTION PATHWAY
**Given Information**:
  Metabolic acidosis indicates a decrease in blood bicarbonate levels.
  Reduced bicarbonate reabsorption in the proximal convoluted tubule is the primary issue.
**Method & Steps**:
 1. Identify the primary mechanism of bicarbonate reabsorption in the proximal tubule.
  Reasoning: Bicarbonate reabsorption is intricately linked to sodium reabsorption.  The Na+/H+ exchanger (NHE3) plays a crucial role in this process.
  Formula/Rule:  $HCO_3^- + H^+ ightarrow H_2CO_3 ightarrow CO_2 + H_2O$
 2. Analyze the role of each transporter option.
  Working: 
            * **Na+/H+ exchanger (NHE3):** This exchanger is located on the luminal membrane of the proximal tubule cells. It exchanges intracellular H+ for luminal Na+. The secreted H+ combines with filtered bicarbonate ($HCO_3^-$) to form carbonic acid ($H_2CO_3$), which then dissociates into $CO_2$ and $H_2O$. $CO_2$ diffuses into the cell and is converted back to $H_2CO_3$ by carbonic anhydrase.  $H_2CO_3$ then dissociates into $H^+$ and $HCO_3^-$. The $HCO_3^-$ is transported into the blood.  A malfunction in NHE3 directly impairs this crucial step.
            * **Na+/K+ ATPase pump:** This pump maintains the electrochemical gradient necessary for other transporters, but it's not directly involved in bicarbonate reabsorption.
            * **Na+/glucose cotransporter (SGLT2):** This transporter is involved in glucose reabsorption, not bicarbonate reabsorption.
            * **Aquaporin-1:** This is a water channel, unrelated to bicarbonate transport.
 3. Conclude the most likely malfunctioning transporter.
  Result: The Na+/H+ exchanger (NHE3) is the most likely malfunctioning transporter because its dysfunction directly prevents the initial step of bicarbonate reabsorption.
  Verification: The clinical presentation of metabolic acidosis with reduced bicarbonate reabsorption strongly points towards a defect in the primary mechanism of bicarbonate reabsorption, which is mediated by NHE3.
 ## OPTION ANALYSIS
A ✓: Na+/H+ exchanger (NHE3) is the primary transporter responsible for bicarbonate reabsorption in the proximal tubule. Its malfunction directly leads to reduced bicarbonate reabsorption and metabolic acidosis.
B :x: The Na+/K+ ATPase pump is essential for maintaining the electrochemical gradient, but it doesn't directly participate in bicarbonate transport.
C :x: The Na+/glucose cotransporter (SGLT2) is involved in glucose reabsorption and is irrelevant to bicarbonate handling.
D :x: Aquaporin-1 facilitates water transport, not bicarbonate transport.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the roles of different proximal tubule transporters: A thorough understanding of each transporter's function is crucial.
   [How to avoid]: Review the functions of each transporter in detail and understand their roles in different processes.
  2.Overlooking the primary mechanism of bicarbonate reabsorption: Focusing on secondary effects instead of the main process.
   [How to avoid]: Always consider the fundamental mechanisms and how they can be affected.
 **Quick Tips**:
  1.Remember the link between Na+ reabsorption and H+ secretion in bicarbonate reabsorption.
  2.Metabolic acidosis with reduced bicarbonate reabsorption strongly suggests a problem with the Na+/H+ exchanger (NHE3).

Mr. BALACHANDRA.N · Answer

The correct Option is "Na+/H+ exchanger (NHE3)"

Mr. BALACHANDRA.N · Answer

Na+/K+ ATPase pump

Mr. BALACHANDRA.N · Answer

Na+/glucose cotransporter (SGLT2)

Mr. BALACHANDRA.N · Answer

Aquaporin-1

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