Renal Physiology And Filtration
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USMLE Step 1 › Renal Physiology And Filtration
A 64-year-old woman with heart failure is treated aggressively with loop diuretics and has dizziness and oliguria. BP 88/54 mmHg. BUN 54 mg/dL, creatinine 2.0 mg/dL. FeNa 0.3%. What is the next step in managing this patient's renal dysfunction?
Give mannitol to increase tubular obstruction and improve urine output
Administer isotonic IV fluids and reduce diuretic dose to restore effective arterial blood volume
Start high-dose furosemide to convert prerenal azotemia to intrinsic renal failure
Start ACE inhibitor immediately to increase efferent arteriolar tone and raise GFR
Begin emergent hemodialysis solely due to elevated BUN/creatinine ratio
Explanation
This question tests understanding of renal physiology and filtration mechanisms (USMLE Step 1). Renal filtration involves the glomerulus where blood pressure and oncotic pressure affect GFR. In this scenario, the patient's lab results and clinical presentation indicate prerenal azotemia from overdiuresis in heart failure. The correct answer (Choice A) is supported by restoring volume to improve perfusion and GFR, explaining the patient's oliguria and low FeNa. Choice B is incorrect because it assumes converting to intrinsic failure with more diuretics, a common error when students overlook volume restoration in prerenal states. Teaching strategies include emphasizing the role of pressure dynamics in GFR regulation and practicing with clinical scenarios to apply physiological principles. Reinforce understanding of renal pathophysiology through case-based learning.
A 58-year-old man with diabetes and hypertension has eGFR 28 mL/min/1.73 m² and potassium 5.8 mEq/L. ECG shows peaked T waves. Which of the following interventions is most appropriate given the renal function test results?
Administer potassium chloride to correct presumed intracellular potassium depletion
Give NSAIDs to increase renal blood flow by afferent dilation and increase potassium excretion
Administer normal saline bolus only, because hyperkalemia is due to dehydration
Start spironolactone to enhance potassium excretion in the collecting duct
Administer IV calcium gluconate to stabilize cardiac membranes, then shift potassium intracellularly
Explanation
This question tests understanding of renal physiology and filtration mechanisms (USMLE Step 1). Renal filtration involves the glomerulus where blood pressure and oncotic pressure affect GFR. In this scenario, the patient's lab results and clinical presentation indicate hyperkalemia in CKD with cardiac effects. The correct answer (Choice A) is supported by stabilizing membranes and shifting potassium, appropriate for low eGFR and ECG changes. Choice B is incorrect because it assumes hyperkalemia is solely from dehydration, a common error when students overlook CKD's impaired potassium excretion. Teaching strategies include emphasizing the role of pressure dynamics in GFR regulation and practicing with clinical scenarios to apply physiological principles. Reinforce understanding of renal pathophysiology through case-based learning.
A 68-year-old man presents with 1 day of oliguria after vomiting and diarrhea. BP 92/56 mmHg, dry mucous membranes. BUN 68 mg/dL, creatinine 2.4 mg/dL, FeNa 0.4%, UA bland. Which mechanism best explains the patient's decreased GFR?
Afferent arteriole dilation from prostaglandin excess increases glomerular capillary hydrostatic pressure
Increased Bowman's space oncotic pressure opposes filtration and lowers net filtration pressure
Efferent arteriole dilation from angiotensin II increases glomerular capillary hydrostatic pressure
Decreased renal perfusion lowers glomerular capillary hydrostatic pressure, reducing net filtration pressure
Increased filtration coefficient (Kf) from podocyte injury increases net filtration pressure
Explanation
This question tests understanding of renal physiology and filtration mechanisms (USMLE Step 1). Renal filtration involves the glomerulus where blood pressure and oncotic pressure affect GFR. In this scenario, the patient's lab results and clinical presentation indicate prerenal azotemia due to hypovolemia. The correct answer (Choice B) is supported by the principle of reduced renal perfusion decreasing glomerular hydrostatic pressure, explaining the patient's oliguria and elevated creatinine. Choice A is incorrect because it assumes prostaglandin excess causes afferent dilation, a common error when students overlook the role of decreased perfusion in prerenal states. Teaching strategies include emphasizing the role of pressure dynamics in GFR regulation and practicing with clinical scenarios to apply physiological principles. Reinforce understanding of renal pathophysiology through case-based learning.
A 62-year-old woman with diabetes has persistent albuminuria and eGFR 42 mL/min/1.73 m². BP 156/94 mmHg. Which of the following interventions is most appropriate given the renal function test results?
Start acetazolamide to increase proximal bicarbonate loss and reduce proteinuria
Start NSAIDs to increase afferent dilation and improve GFR long term
Start an ACE inhibitor or ARB to reduce intraglomerular pressure and slow progression of proteinuric CKD
Start loop diuretic solely to reduce albuminuria by increasing Kf
Start high-protein diet to increase filtration fraction and raise eGFR
Explanation
This question tests understanding of renal physiology and filtration mechanisms (USMLE Step 1). Renal filtration involves the glomerulus where blood pressure and oncotic pressure affect GFR. In this scenario, the patient's lab results and clinical presentation indicate diabetic CKD with proteinuria and hypertension. The correct answer (Choice A) is supported by reducing efferent tone to lower glomerular pressure, explaining the benefit in slowing CKD progression. Choice B is incorrect because it assumes NSAIDs improve GFR long-term, a common error when students overlook their risk in CKD. Teaching strategies include emphasizing the role of pressure dynamics in GFR regulation and practicing with clinical scenarios to apply physiological principles. Reinforce understanding of renal pathophysiology through case-based learning.
A 57-year-old man has longstanding hypertension and now has progressive CKD. Which intervention is most appropriate to slow further decline in GFR?
High-protein diet to increase single-nephron GFR and prevent nephron loss
Start thiazide diuretic solely to increase albumin filtration and reduce edema
Stop all antihypertensives to allow higher renal perfusion pressure and increase GFR
Tight blood pressure control with ACE inhibitor or ARB if tolerated to reduce intraglomerular hypertension
Routine NSAID use to increase renal perfusion and preserve GFR
Explanation
This question tests understanding of renal physiology and filtration mechanisms (USMLE Step 1). Renal filtration involves the glomerulus where blood pressure and oncotic pressure affect GFR. In this scenario, the patient's lab results and clinical presentation indicate hypertensive CKD progression. The correct answer (Choice A) is supported by controlling glomerular hypertension to preserve nephrons, explaining the benefit in slowing GFR decline. Choice B is incorrect because it assumes NSAIDs preserve GFR, a common error when students overlook their risks in CKD. Teaching strategies include emphasizing the role of pressure dynamics in GFR regulation and practicing with clinical scenarios to apply physiological principles. Reinforce understanding of renal pathophysiology through case-based learning.