A hospitalized patient has been hypotensive from sepsis for 6 hours. Urine output drops to 5 mL/hr, creatinine rises, and urinalysis shows muddy brown granular casts with FENa of 3.5%. What is the most likely diagnosis and the key evidence for it?
APrerenal azotemia, because sepsis reduces renal perfusion
BPrerenal azotemia, because hypotension always precedes the oliguria
CAcute tubular necrosis, because elevated FENa and muddy brown casts indicate direct tubular damage
DAcute tubular necrosis, because all septic patients develop ATN
The combination of muddy brown granular casts (sloughed tubular epithelial cells indicating structural damage) and FENa >2% (damaged tubules unable to reabsorb sodium) distinguishes ATN from prerenal azotemia. In prerenal AKI, intact tubules avidly conserve sodium (FENa <1%) and produce no cellular casts. The hypotension is the cause of the ATN, not evidence against it — prolonged ischemia has crossed the threshold from reversible prerenal AKI to irreversible tubular injury.
Question 2 Multiple Choice
During the maintenance phase of ATN, why might a patient require dialysis even though the original hypotension has been corrected?
ATubular cells are actively dying and releasing toxins that must be cleared
BSurviving tubular cells are dysfunctional and GFR remains depressed despite restored perfusion
CThe kidneys enter irreversible failure during the maintenance phase
DDialysis removes nephrotoxins that continue accumulating in tubular cells
The maintenance phase is a plateau of dysfunction: the initial injury has occurred, the trigger is removed, but surviving tubular cells — though alive — are not yet functional. They cannot properly reabsorb solutes or concentrate urine, GFR remains depressed, and fluid and electrolyte imbalance may require dialysis support. The critical distinction is that this phase is temporary: it precedes the recovery phase, in which surviving cells regenerate and restore function.
Question 3 True / False
In prerenal AKI, the fractional excretion of sodium (FENa) is typically greater than 2%, whereas in ATN it is less than 1%.
TTrue
FFalse
Answer: False
This is reversed. In prerenal AKI, intact tubules avidly conserve sodium to restore intravascular volume — FENa is <1%. In ATN, damaged tubular cells cannot perform normal sodium reabsorption, so sodium spills into the urine — FENa rises above 2%. The FENa is a functional readout of tubular integrity: low FENa means tubules are working; high FENa means they are not.
Question 4 True / False
Most cases of ATN resolve with spontaneous tubular regeneration and near-full recovery of renal function.
TTrue
FFalse
Answer: True
Unlike many organs, the renal tubular epithelium has significant regenerative capacity. Surviving cells dedifferentiate, proliferate, and re-epithelialize damaged segments during the recovery phase. This is why ATN management is primarily supportive — remove the offending agent, restore hemodynamics, bridge with dialysis if needed — and time. Patients with baseline CKD, prolonged ischemia, or severe insults have worse recovery trajectories, but the majority of otherwise healthy patients recover near-baseline function.
Question 5 Short Answer
A patient with mild dehydration has elevated creatinine and oliguria that normalizes within 24 hours after IV fluids. A second patient with severe septic shock has similar initial labs but does not improve with fluids. Explain the pathophysiological difference.
Think about your answer, then reveal below.
Model answer: The first patient has prerenal azotemia — reduced perfusion causes reversible oliguria, but tubular cells are structurally intact. Restoring perfusion immediately restores function. The second patient has progressed to ischemic ATN — prolonged ATP depletion has killed proximal tubular and thick ascending limb epithelial cells. Fluids cannot restore function to dead cells; recovery requires tubular regeneration, which takes days to weeks. The threshold between reversible prerenal AKI and irreversible ATN is crossed when ischemia is severe and sustained enough to deplete ATP and trigger cell death.
This distinction drives management: prerenal AKI responds to fluid resuscitation; ATN does not, and aggressive fluid administration in an oliguric ATN patient risks fluid overload. The urine sediment and FENa help distinguish them, but the clinical trajectory — response or non-response to fluids — is itself diagnostic.