A 68-year-old man is brought to the emergency department with complaints of dizziness and decreased urine output over the past two days. He has a history of hypertension and type 2 diabetes mellitus. He takes lisinopril, hydrochlorothiazide, metformin, and glipizide. His blood pressure is 90/60 mmHg, and his heart rate is 110 beats per minute. On physical examination, he appears dehydrated with dry mucous membranes and poor skin turgor. Laboratory results show a blood urea nitrogen (BUN) level of 60 mg/dL and a serum creatinine level of 2.5 mg/dL. Urinalysis is bland. Calculation of the fractional excretion of sodium (FeNa) of 0.5%.
Which of the following is the most likely diagnosis?
The correct answer is:
E) Prerenal acute kidney injury
The most common cause of prerenal acute kidney injury is hypovolemia. This patient's presentation of dehydration, elevated BUN-to-creatinine ratio greater than 20 (60/2.5 = 24), and a fractional excretion of sodium (FeNa) less than 1% (0.5%) are indicative of prerenal acute kidney injury as the kidneys try to retain sodium.
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 usually show a high BUN-to-creatinine ratio or low FeNa.
Answer choice B: Acute tubular necrosis, is incorrect. Acute tubular necrosis often presents with a FeNa greater than 2% and a BUN-to-creatinine ratio closer to 10-15, typically following ischemic or nephrotoxic injury.
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.
Key Learning Point
The most common cause of prerenal acute kidney injury is hypovolemia. Look for a BUN-to-creatinine ratio greater than 20 and a fractional excretion of sodium (FeNa) less than 1% as the kidneys try to retain sodium.
