What is the likely cause of blood urea nitrogen concentration elevated in acute renal failure?

Appeared in: KPSC Staff Nurse - 2016

Explanation

Reduced renal blood flow (prerenal state) decreases the glomerular filtration rate (GFR).
A lower GFR leads to slower movement of filtrate through the tubules, allowing for increased reabsorption of urea back into the blood.
Creatinine is not reabsorbed as readily as urea, causing a disproportionate rise in BUN compared to creatinine.
This mechanism results in an elevated BUN-to-creatinine ratio, often greater than 20:1, which is a hallmark of prerenal azotemia.

Option A: Fluid retention is a consequence, not a cause, of acute renal failure. The kidneys' inability to excrete water leads to fluid overload.
Option B: While massive hemolysis can lead to intrinsic acute kidney injury by causing tubular damage, it is not the direct mechanism for the disproportionate BUN elevation seen in prerenal states.
Option C: A below-normal metabolic rate (e.g., in hypothyroidism or starvation) would decrease protein catabolism, leading to lower production of urea and thus a lower BUN level.

Clinical Relevance

Nursing practice connection: Knowing Pathophysiology of elevated BUN in prerenal acute renal failure helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses must assess for signs of prerenal azotemia by monitoring blood pressure, heart rate, urine output, and signs of dehydration or volume loss. A high BUN:Cr ratio is a critical lab value to note.
Interventions for prerenal AKI focus on restoring renal perfusion. This may involve administering IV fluids for dehydration, blood products for hemorrhage, or vasopressors for shock, as ordered.
What if? If a patient with AKI has a BUN:Cr ratio of 15:1 with both BUN and creatinine elevated, the cause is more likely intrinsic renal damage (like acute tubular necrosis) rather than a prerenal issue. The nursing focus would shift to identifying and removing nephrotoxins and supporting kidney function, as fluid resuscitation alone may not be effective and could cause overload.

How to Approach the Question

First, identify the core concept being tested: the pathophysiology of acute renal failure (ARF) and the significance of blood urea nitrogen (BUN).
Analyze the question stem, which asks for the cause of an elevated BUN in ARF.
Evaluate each option based on your knowledge of renal physiology.
Option A (Fluid retention) is a result of ARF, not a cause of the BUN elevation itself.
Option B (Hemolysis) can cause a type of ARF, but doesn't explain the specific mechanism of a disproportionate BUN rise.
Option C (Below normal metabolic rate) would logically lead to decreased urea production, so it's incorrect.

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 2 pp. 264-266, 283-285

Practise the full KPSC Staff Nurse - 2016

Attempt every question from this paper in a timed mock, then review the full solution for each one.