Acute Kidney Injury (AKI)

Why Pre-renal AKI is correct

• BUN:Cr ratio >20:1 (here 25:1) reflects urea reabsorption in the setting of reduced renal perfusion
• Fractional excretion of sodium (FENa) <1% (here 0.5%) indicates intact tubular sodium reabsorption, consistent with pre-renal state
• Urine sodium <20 mEq/L (here 15 mEq/L) further supports pre-renal AKI
• Additional pre-renal features: urine osmolality >500 (concentrating ability preserved) and bland urinary sediment
• Causes include volume depletion, sepsis, heart failure, hepatorenal syndrome

Why the others are wrong

• Acute interstitial nephritis — Acute interstitial nephritis (AIN) is most often drug-induced (NSAIDs, proton pump inhibitors (PPIs), antibiotics especially beta-lactams); fever, rash, or eosinophilia may occur, but the classic triad is present in only about 10% of cases; urine eosinophils are unreliable for diagnosing it (about 31% sensitive and 68% specific); FENa would typically be elevated
• Post-renal AKI from urinary obstruction — Post-renal AKI presents with hydronephrosis on ultrasound; indices may vary but the urinary findings here do not suggest obstruction
• Intrinsic AKI from acute tubular necrosis — Acute tubular necrosis (ATN) shows BUN:Cr 10-15:1, FENa >2%, urine Na >40 mEq/L, urine osmolality <350, and muddy brown granular casts — the opposite of this patient's findings
• Intrinsic AKI from atheroembolic disease — his age fits, but cholesterol emboli typically follow arterial catheterization, vascular surgery, or anticoagulation and cause a stepwise decline over weeks with livedo reticularis and eosinophilia; this rise during pneumonia with a high BUN:Cr ratio, low FENa, and low urine sodium is the hypoperfusion pattern of pre-renal AKI

Additional high-yield points

• Pre-renal AKI can progress to ATN if the underlying cause is not treated
• Treatment: address the underlying cause; pre-renal AKI from volume depletion responds to fluids, but pre-renal AKI from heart failure or hepatorenal syndrome does not; ATN is managed supportively

Why Prerenal acute kidney injury is correct

• Prerenal AKI results from renal hypoperfusion (here from hypovolemia); the tubules are intact and respond to angiotensin II, aldosterone, and ADH by avidly reabsorbing sodium and water
• Characteristic lab pattern: urine sodium <20 mEq/L, fractional excretion of sodium (FENa) <1%, urine osmolality >500 mOsm/kg (concentrated), and a disproportionate rise in BUN relative to creatinine (>20:1) because reabsorbed urea follows water
• Bland urine with only hyaline casts is expected in prerenal AKI
• This patient's values (urine Na 12 mEq/L, FENa 0.6%, urine osm 580 mOsm/kg, BUN:Cr 28:1) all fit the prerenal pattern

Why the others are wrong

• Acute tubular necrosis — Injured tubules cannot reabsorb sodium or concentrate urine, producing urine Na >40, FENa >2%, isosthenuric urine osmolality near 300 mOsm/kg, BUN:Cr around 10–15:1, and muddy-brown granular casts
• Postrenal obstructive injury — Postrenal AKI is suggested by hydronephrosis on imaging and usually does not produce this avid-sodium-retention index pattern, although early obstruction can; the vomiting, poor intake, and orthostasis here point instead to volume depletion
• Acute glomerulonephritis — Glomerulonephritis causes an active sediment with dysmorphic RBCs, RBC casts, and proteinuria rather than the bland urine seen here; acute GN can also produce a low FENa, so the sediment, not the sodium indices, is what excludes it
• Acute interstitial nephritis — drug-induced interstitial inflammation is a common intrinsic cause of AKI in older adults and its FENa is variable, but tubulointerstitial injury tends to impair urinary concentration and often produces sterile pyuria, white blood cell casts, or low-grade proteinuria; the bland sediment, concentrated urine, and vomiting with orthostasis point instead to volume depletion