Question: Respiratory & Renal Systems
A 70-year-old man with longstanding congestive heart failure has had decreasing urine output and fatigue over 2 days. He stopped taking his medications a week ago. His blood pressure is 120/80 mm Hg. Laboratory studies show a serum urea nitrogen of 40 mg/dL and creatinine of 2.0 mg/dL (ratio 20:1), a urine sodium of 5 mEq/L, and a urine specific gravity of 1.025. Which of the following best explains these findings?
| BUN | 40 mg/dL |
|---|---|
| Creatinine | 2.0 mg/dL |
| Urine sodium | 5 mEq/L |
Choose one answer.
Explanation
Correct answer: D. Prerenal azotemia from decreased renal perfusion
A high BUN-to-creatinine ratio (about 20:1), a very low urine sodium, and concentrated urine indicate prerenal azotemia. Reduced cardiac output lowers renal blood flow, so the kidney avidly reabsorbs sodium and water and reabsorbs urea along with it. The tubules are intact, distinguishing this from acute tubular necrosis, in which urine sodium is high and the urine is isosthenuric. Interstitial nephritis, obstruction, and glomerulonephritis give different urinary findings.
Learning objective: Differentiate prerenal azotemia from intrinsic renal failure using urinary indices.
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