A 42-year-old woman is admitted with severe sepsis secondary to urosepsis. She receives aggressive fluid resuscitation and broad-spectrum antibiotics. On day 3 of admission, her serum creatinine has risen from 0.9 mg/dL to 2.6 mg/dL despite adequate fluid resuscitation and blood pressure support with noradrenaline. Urine output is 0.4 mL/kg/hr. Renal ultrasound shows normal-sized kidneys with no obstruction. Urinalysis reveals muddy brown casts and coarse granular casts; urine osmolality is 350 mOsm/L. Which of the following histopathologic findings would be expected on renal biopsy in this patient?

Explanation

## Histopathology of Acute Tubular Necrosis **Key Point:** ATN is characterized by acute injury to tubular epithelial cells with preservation of the basement membrane. This is the critical distinguishing feature—the basement membrane scaffold remains intact, allowing for complete functional recovery without permanent scarring. [cite:Robbins 10e Ch 20] ### Microscopic Features of ATN | Feature | Finding | Significance | |---------|---------|---------------| | **Tubular epithelium** | Focal to patchy necrosis and sloughing | Cells undergo apoptosis and necrosis; debris fills tubular lumen | | **Basement membrane** | Intact, non-disrupted | Preserved architecture allows regeneration and complete recovery | | **Cellular regeneration** | Mitotic figures at margins; flattened epithelium | Tubular cells re-epithelialize within days to weeks | | **Luminal contents** | Muddy brown casts, cellular debris, pigment | Reflects necrotic epithelial cells and myoglobin/hemoglobin | | **Glomeruli** | Normal or minimally affected | Glomerular filtration barrier is intact | | **Interstitium** | Minimal edema, sparse inflammatory infiltrate | Unlike acute interstitial nephritis (dense infiltration) | ### Electron Microscopy Findings **High-Yield:** On transmission electron microscopy (TEM), ATN shows: - Loss of brush border microvilli in proximal tubules - Mitochondrial swelling and cristal disruption (reflecting ATP depletion) - Widening of intercellular spaces and disruption of tight junctions - Intact basement membrane with no electron-dense deposits ### Phases of Tubular Injury and Recovery ```mermaid flowchart TD A[Ischemic insult<br/>ATP depletion]:::urgent --> B[Acute phase<br/>Day 0-3]:::outcome B --> C[Epithelial necrosis<br/>Loss of microvilli<br/>Cell sloughing]:::action C --> D[Early recovery<br/>Day 3-7]:::action D --> E[Regeneration phase<br/>Mitosis at margins<br/>Re-epithelialization]:::action E --> F[Late recovery<br/>Week 2-4]:::action F --> G[Complete restoration<br/>Basement membrane intact<br/>Full functional recovery]:::outcome ``` **Clinical Pearl:** The intact basement membrane is why ATN, unlike acute cortical necrosis or RPGN, has the potential for complete functional recovery. Most patients recover normal or near-normal renal function within 3–5 weeks, even after severe ATN. **Mnemonic: TUBULAR NECROSIS = Tubules damaged, Basement membrane preserved, Uniform recovery expected, Lumen filled with debris, Apoptosis/necrosis of epithelium, Regeneration occurs at margins** ## Why the Correct Answer is Correct The clinical presentation (sepsis-induced AKI, muddy brown casts, normal ultrasound, preserved renal function potential) combined with the histologic hallmark of ATN—focal tubular necrosis with **preserved basement membrane**—makes this the correct answer. The intact basement membrane distinguishes ATN from cortical necrosis and allows for complete recovery.