A 58-year-old man with diabetes mellitus presents with acute kidney injury following a contrast-enhanced CT angiography for suspected pulmonary embolism. His urine microscopy shows muddy brown casts and granular casts. Which histopathological feature best distinguishes acute tubular necrosis (ATN) from acute interstitial nephritis (AIN)?

Question

Accepted Answer

D. Preservation of tubular basement membrane with epithelial cell necrosis. ## Histopathological Distinction Between ATN and AIN

**Key Point:** The hallmark of ATN is loss of tubular epithelial cell integrity with preservation of the tubular basement membrane (TBM), whereas AIN is characterized by interstitial inflammation without primary tubular epithelial necrosis.

### Comparison Table

| Feature | ATN | AIN |
| --- | --- | --- |
| **Tubular epithelium** | Necrosis and sloughing | Intact or minimal injury |
| **Tubular basement membrane** | Preserved | Preserved |
| **Interstitium** | Minimal edema, sparse inflammation | Marked edema, dense lymphocytic infiltrate |
| **Glomeruli** | Normal | Normal |
| **Electron microscopy** | Loss of brush border, mitochondrial swelling | Interstitial edema, no tubular changes |
| **Urine findings** | Muddy brown/granular casts, epithelial cells | Eosinophiluria (if drug-induced), WBC casts |

**High-Yield:** In ATN, the tubular basement membrane remains intact—this is the critical discriminator. The epithelial cells are damaged and sloughed, but the structural scaffold persists, allowing regeneration. In contrast, AIN involves immune-mediated interstitial inflammation without primary tubular epithelial injury.

### Clinical Pearl

The presence of **muddy brown casts** (as in this patient) is pathognomonic for ATN and reflects sloughed tubular epithelial cells and pigments (myoglobin or hemoglobin). This finding, combined with preserved TBM on biopsy, confirms ATN over AIN.

### Mechanism

ATN results from ischemic or nephrotoxic injury (in this case, contrast-induced) that causes:
1. Loss of tubular epithelial cell polarity and tight junctions
2. Mitochondrial dysfunction and ATP depletion
3. Epithelial cell death and detachment
4. BUT the basement membrane scaffold remains intact for regeneration

AIN, by contrast, is immune-mediated (drugs, infections, autoimmune) and primarily targets the interstitium with T-cell infiltration.

Answer

A. Predominant interstitial edema with lymphocytic infiltration

Answer

B. Glomerular capillary loop thickening

Answer

C. Crescent formation with fibrinoid necrosis

Explanation

Histopathological Distinction Between ATN and AIN

Comparison Table

Clinical Pearl

Mechanism

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Feature	ATN	AIN
Tubular epithelium	Necrosis and sloughing	Intact or minimal injury
Tubular basement membrane	Preserved	Preserved
Interstitium	Minimal edema, sparse inflammation	Marked edema, dense lymphocytic infiltrate
Glomeruli	Normal	Normal
Electron microscopy	Loss of brush border, mitochondrial swelling	Interstitial edema, no tubular changes
Urine findings	Muddy brown/granular casts, epithelial cells	Eosinophiluria (if drug-induced), WBC casts