Obligatory water reabsorption takes place at

Options

A. PCT
B. DCT
C. Collecting duct
D. Both (A) and (B)

Answer

To solve the question "Obligatory water reabsorption takes place at," we need to analyze the options provided and understand the process of water reabsorption in the nephron of the kidney.

### Step-by-Step Solution:

1. **Understanding Obligatory Water Reabsorption**:
- Obligatory water reabsorption refers to the process where water is reabsorbed from the filtrate back into the blood regardless of the body's hydration status. This is essential for maintaining fluid balance in the body.

2. **Identifying the Nephron Structures**:
- The nephron consists of several parts: the Proximal Convoluted Tubule (PCT), Distal Convoluted Tubule (DCT), and the Collecting Duct. Each part has a different role in water and solute reabsorption.

3. **Analyzing the Options**:
- **Option 1: PCT (Proximal Convoluted Tubule)**:
- The PCT is responsible for the majority of water reabsorption. Approximately 65-70% of the water is reabsorbed here, and this process is obligatory.
- **Option 2: DCT (Distal Convoluted Tubule)**:
- The DCT does reabsorb water, but this is primarily under hormonal control (facultative reabsorption), and only about 10-15% of water is reabsorbed here.
- **Option 3: Collecting Duct**:
- The Collecting Duct also primarily engages in facultative water reabsorption, which is influenced by the hormone ADH (antidiuretic hormone). Thus, it is not the site of obligatory water reabsorption.
- **Option 4: Both A and B**:
- Since only the PCT is responsible for obligatory water reabsorption, this option is incorrect.

4. **Conclusion**:
- Based on the analysis, the correct answer is **Option 1: PCT (Proximal Convoluted Tubule)**, where obligatory water reabsorption occurs.

### Final Answer:
**Obligatory water reabsorption takes place at the Proximal Convoluted Tubule (PCT).**

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