What histopathologic findings define acute tubular necrosis?

• A. Glomerular capillary collapse and mesangial expansion.
• B. Interstitial fibrosis and scarring.
• C. Tubular epithelial cell necrosis, loss of brush border, tubular dilation, and pigmented casts.
• D. Tubular thinning with preserved function.

Answer: (C)

Acute tubular necrosis is defined by injury to the renal tubules with damage to the tubular epithelium. Histologically, you see necrosis of tubular epithelial cells, loss of the apical brush border (especially in the proximal tubule), detachment of cells with desquamation into the tubular lumen, tubular dilation, and the formation of intratubular casts. The pigmented or muddy brown casts reflect sloughed epithelial cells and tubular proteins accumulating within the tubules. This pattern matches an acute, tubular injury from ischemia or nephrotoxins.

The other patterns don’t fit ATN: glomerular capillary collapse with mesangial expansion points to glomerular diseases, not tubular injury; interstitial fibrosis and scarring indicates chronic damage; tubular thinning with preserved function suggests minimal acute tubular damage.