During the maintenance phase of acute tubular necrosis, which urine finding is most typical?

• A. Muddy brown granular casts
• B. Red blood cell casts
• C. WBC casts
• D. Epithelial cell casts

Answer: (A)

During the maintenance phase of acute tubular necrosis, the urine typically shows muddy brown granular casts. These casts form from sloughed tubular epithelial cells and proteinaceous material that accumulate in the tubules as the tubular lining remains necrotic. The granular, pigmented appearance is a hallmark of ATN and reflects ongoing tubular injury and obstruction, which is why this finding is most typical in this phase.

Red blood cell casts point toward glomerular disease or vasculitis, not the tubular injury pattern of ATN. White blood cell casts suggest interstitial nephritis or pyelonephritis. Epithelial cell casts can occur in ATN but are less specific than the classic muddy brown granular casts, which best distinguish ATN from other renal pathologies.