A 58-year-old man with diabetes mellitus type 2 and hypertension presents to the emergency department with a 3-day history of fever, cough, and dyspnea. On examination, he is hypotensive (BP 88/54 mmHg), tachycardic (HR 118/min), and has bilateral crackles on auscultation. Laboratory investigations show: serum creatinine 3.2 mg/dL (baseline 1.0 mg/dL), BUN 68 mg/dL, urine output 200 mL in 24 hours, and urine sodium 8 mEq/L. Urinalysis shows muddy brown casts and proteinuria 2+. What is the most likely diagnosis?

Explanation

## Clinical Analysis ### Key Presentation Features **Key Point:** This patient has acute kidney injury (AKI) with clinical and laboratory evidence pointing to acute tubular necrosis (ATN). ### Diagnostic Evidence | Feature | Finding | Significance | |---------|---------|---------------| | **Clinical context** | Sepsis (fever, hypotension, tachycardia, respiratory infection) | ATN most common cause in ICU/sepsis | | **Urine sodium** | 8 mEq/L | LOW — suggests tubular damage, not prerenal | | **Muddy brown casts** | Present | Pathognomonic for ATN (epithelial cell casts) | | **Urine output** | 200 mL/24 hrs (oliguric) | Consistent with ATN | | **Proteinuria** | 2+ | Suggests glomerular involvement but non-specific | | **Creatinine rise** | 3.2 from baseline 1.0 | Acute, significant elevation | ### Mechanism of ATN in Sepsis 1. Sepsis → systemic inflammation + endotoxemia 2. Renal hypoperfusion + direct tubular toxin injury 3. Loss of tubular epithelial cell integrity 4. Muddy brown casts (sloughed epithelial cells + hemoglobin/myoglobin) 5. Inability to concentrate urine → high urine sodium despite low GFR **High-Yield:** The combination of **muddy brown casts + low urine sodium + sepsis** is diagnostic for ATN. ### Why ATN, Not Prerenal? **Clinical Pearl:** Prerenal AKI typically shows urine sodium <20 mEq/L AND fractional excretion of sodium (FENa) <1%. However, **muddy brown casts are NOT seen in prerenal AKI** — they indicate tubular epithelial damage. The presence of muddy brown casts definitively excludes prerenal disease. **Mnemonic: MUDDY BROWN = ATN** — Muddy brown casts = tubular necrosis, not just functional renal hypoperfusion. [cite:Harrison 21e Ch 297]