Acute tubular necrosis is characterized by which of the following urine findings and serum changes?

• A. Hematuria with red cell casts and hypokalemia
• B. Epithelial cell casts and muddy brown casts; low specific gravity; hyperkalemia and hyperphosphatemia
• C. Waxy casts with hypokalemia
• D. Red blood cell casts with metabolic alkalosis

Answer: (B)

Acute tubular necrosis is an intrinsic renal injury where tubular epithelial cells are damaged, leading to sloughing of cells into the urine and granular casts. The hallmark urine finding is muddy brown granular casts formed from these tubular debris, often with epithelial cell casts. Because the tubules can’t concentrate urine properly, the urine specific gravity remains relatively low despite oliguria. The damaged tubules also fail to excrete potassium and phosphate effectively, causing hyperkalemia and hyperphosphatemia in the serum. This combination of epithelial and muddy brown casts, low urine concentrating ability, and electrolyte abnormalities is characteristic of ATN.