In acute tubular necrosis, which segment of the nephron is MOST commonly affected?

Explanation

## Segmental Distribution of Tubular Necrosis in ATN **Key Point:** ATN predominantly affects the **S3 segment of the proximal tubule** and the **thick ascending limb (TAL)** of the loop of Henle. These are the most metabolically active segments and most vulnerable to ischemic and nephrotoxic injury. ### Why S3 and TAL Are Most Vulnerable | Segment | Metabolic Activity | O₂ Dependency | Vulnerability | |---------|-------------------|---------------|---------------| | **S3 proximal tubule** | Very high | High | **Most vulnerable** | | **Thick ascending limb** | High (Na-K-2Cl pump) | High | **Most vulnerable** | | **S1–S2 proximal tubule** | High | High | Moderately vulnerable | | **Distal tubule** | Moderate | Moderate | Less vulnerable | | **Collecting duct** | Moderate | Moderate | Less vulnerable | **High-Yield:** The S3 segment has the highest density of mitochondria and relies heavily on oxidative phosphorylation. In ischemia or hypoxia, this segment is the first to undergo necrosis. **Mnemonic:** **"S3 and TAL Take the Hit"** — The most metabolically demanding segments suffer first in ATN. ### Clinical Correlation The selective injury to S3 and TAL explains: - Loss of concentrating ability (TAL dysfunction) - Impaired active transport (S3 dysfunction) - Polyuric phase of ATN recovery (regenerating tubules initially lack full function)