Which are the phases of acute tubular necrosis and a characteristic urine finding during the maintenance phase?

• A. Pre-renal, intrinsic, post-renal; hyaline casts; FENa <1%
• B. Initiation (insult), maintenance (oliguric), diuretic (recovery); muddy brown granular casts; FENa often >2%
• C. Onset, rapid recovery; RBC casts; FENa variable
• D. Chronic, azotemia; crystals in urine; FENa not applicable

Answer: (B)

Acute tubular necrosis develops in a sequence of stages: initial injury from the insult, a maintenance phase with ongoing tubular dysfunction and oliguria, and then a diuretic (recovery) phase as function begins to return. During the maintenance phase, the damaged tubular cells slough off and obstructive debris accumulates, leading to a urine sediment that commonly shows muddy brown granular casts, the telltale sign of tubular injury. The kidneys lose their ability to reclaim sodium, so the fractional excretion of sodium rises, typically exceeding 2%, reflecting salt wasting despite a low urine output. This combination—oliguria with granular casts and a high FENa—is characteristic of ATN in the maintenance stage. In contrast, features like hyaline casts with a low FENa point to prerenal azotemia from reduced renal perfusion, not ATN, and other options describe patterns that don’t fit the classic ATN phases or urine findings.