🔄 Acute Tubular Necrosis (ATN): The Reversible Renal Emergency
ATN is the most common cause of acute kidney injury (AKI) in hospitalized patients. It’s a high-yield condition tested for its distinctive urinary findings, pathophysiology, and recovery phases.
USMLE Buzzword: “Muddy brown granular casts” = ATN
🧬 Pathophysiology Breakdown
Injury to renal tubular epithelial cells → cell death → tubular obstruction and backleak of filtrate
Two major causes:
Ischemia: prolonged hypotension or shock
Nephrotoxins: aminoglycosides, contrast dye, myoglobin, cisplatin
📉 Phases of ATN
| Phase | Description |
|---|---|
| Initiation | Injury begins; slight drop in urine output |
| Maintenance | Oliguria, rising BUN/Cr, electrolyte abnormalities |
| Recovery | Diuresis with potential fluid/electrolyte loss |
🔬 Diagnostic Features
| Lab/Test | ATN | Prerenal |
|---|---|---|
| Urine Na⁺ | > 40 mEq/L | < 20 mEq/L |
| FENa (%) | > 2% | < 1% |
| Urine osmolality | < 350 | > 500 |
| BUN:Cr ratio | ~15:1 | > 20:1 |
| Urine sediment | Muddy brown casts | Bland or hyaline |
FENa and casts are KEY differentiators for USMLE!
💊 Management Approach
| Step | Treatment |
|---|---|
| 1 | Stop nephrotoxic agents (e.g., aminoglycosides, contrast) |
| 2 | Provide IV fluids and correct electrolytes |
| 3 | Use diuretics only if fluid overloaded |
| 4 | Initiate dialysis if indicated (acidosis, volume overload, uremia) |
📚 Sample USMLE Vignette
A hospitalized patient receives IV contrast for CT scan and develops oliguria. Labs show BUN:Cr = 15:1, FENa = 3%, and muddy brown casts. What’s the likely diagnosis?
✅ Answer: Acute tubular necrosis (ATN)
📲 Call to Action
💧 Don’t let nephron knowledge slip through the cracks.
📲 Download the King of the Curve App for:
Kidney physiology flashcards
Cast recognition visual quizzes
Daily leaderboard questions
Curve Coins for renal mastery
Frequently Asked Questions (FAQs)
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