A 72-year-old man is admitted to the hospital with severe sepsis secondary to a urinary tract infection. Past medical history includes hypertension, hyperlipidemia, benign prostatic hyperplasia, and squamous cell cancer of the skin. Despite aggressive fluid resuscitation, his urine output remains low. Laboratory results show a blood urea nitrogen (BUN) level of 40 mg/dL and a serum creatinine level of 3.0 mg/dL. Urinalysis reveals muddy brown granular casts and free renal tubular epithelial cells. The fractional excretion of sodium (FeNa) is 3%.
B) Acute tubular necrosis
This patient has sepsis with persistence of acute kidney injury (AKI) despite aggressive fluid resuscitation. Acute tubular necrosis (ATN) is the most common cause of intra-renal acute kidney injury. The three major causes of ATN are kidney ischemia, sepsis, and nephrotoxins. The classic urinalysis of ATN reveals muddy brown granular casts and free renal tubular epithelial cells. The fractional excretion of sodium (FeNa) typically is greater than 2% in ATN. Fluid repletion will typically correct pre-renal disease while the AKI will persist in ATN.
Answer choice A: Acute interstitial nephritis, is incorrect. Acute interstitial nephritis typically presents with fever, rash, and eosinophilia, often in response to medications, and does not show muddy brown granular casts or a high FeNa.
Answer choice C: Glomerulonephritis, is incorrect. Glomerulonephritis often presents with hematuria, proteinuria, and red blood cell casts in the urine, which are not seen in this patient.
Answer choice D: Postrenal acute kidney injury, is incorrect. Postrenal acute kidney injury is usually caused by obstruction of the urinary tract and often presents with symptoms of urinary retention or hydronephrosis, which are not present in this case.
Answer choice E: Prerenal acute kidney injury, is incorrect. Prerenal acute kidney injury typically presents with a BUN-to-creatinine ratio greater than 20 and a FeNa less than 1%. Urinalysis typically is normal or near normal with hyaline or fine granular casts that are sometimes seen. Fluid repletion usually corrects prerenal AKI, which is not the case here.
Key Learning Point
Acute tubular necrosis (ATN) is the most common cause of intra-renal acute kidney injury (AKI). The three major causes of ATN are kidney ischemia, sepsis, and nephrotoxins. The classic urinalysis of ATN reveals muddy brown, granular epithelial casts and free renal tubular epithelial cells. The fractional excretion of sodium (FeNa) typically is greater than 2% in ATN. Fluid repletion will typically correct pre-renal disease while the AKI will persist despite fluid resuscitation in ATN.
