A 62-year-old man with diabetes mellitus type 2 and hypertension presents with acute kidney injury. His serum creatinine has risen from baseline 1.2 mg/dL to 3.8 mg/dL over 48 hours. Urine microscopy shows muddy brown casts and epithelial cell casts. Which of the following is NOT a typical feature of acute tubular necrosis (ATN)?

## Acute Tubular Necrosis (ATN) — Urine Findings **Key Point:** ATN is characterized by damage to the tubular epithelium, resulting in a distinctive urinary profile that helps differentiate it from prerenal azotemia. ### Typical Urine Features in ATN | Feature | ATN | Prerenal AKI | |---------|-----|-------------| | **FENa** | >2% (often >3%) | <1% | | **Urine osmolality** | <350 mOsm/kg (dilute) | >500 mOsm/kg (concentrated) | | **Urine specific gravity** | <1.015 | >1.030 | | **Muddy brown casts** | Present | Absent | | **Epithelial cell casts** | Present | Absent | | **Hyaline casts** | Absent or rare | Present | **High-Yield:** Hyaline casts are the most common type of urinary cast and are seen in **prerenal** states, dehydration, and normal individuals. They are NOT characteristic of ATN. **Clinical Pearl:** The presence of muddy brown (granular) casts and epithelial cell casts in the setting of elevated FENa and dilute urine strongly suggests ATN. Hyaline casts, by contrast, indicate prerenal physiology or benign conditions. **Warning:** Do not confuse hyaline casts (benign, seen in prerenal states) with granular/muddy brown casts (pathologic, seen in ATN). This is a common exam trap. ### Why the Other Options Are Correct Features of ATN - **FENa >2%:** Indicates tubular dysfunction; tubules cannot reabsorb filtered sodium because epithelial cells are damaged. - **Urine osmolality <350 mOsm/kg:** Reflects loss of tubular concentrating ability due to epithelial injury. - **Muddy brown casts:** Pathognomonic for ATN; composed of necrotic tubular epithelial cells and hemoglobin/myoglobin. [cite:Harrison 21e Ch 279]