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MBBS QUESTION #10260
Question 1
The filtration fraction is defined as $FF = \dfrac{GFR}{RPF}$ (glomerular filtration rate divided by renal plasma flow). Selective constriction of the efferent arteriole (as with low-dose ACE-inhibitor withdrawal or early diabetic nephropathy) typically produces which change?
  • Both GFR and RPF fall equally, so filtration fraction is unchanged
  • GFR rises (or is relatively preserved) while RPF falls due to increased downstream resistance, so filtration fraction increases✔️
  • Filtration fraction falls because GFR drops much more than RPF
  • Neither GFR nor RPF changes with efferent arteriolar tone
Correct Answer Explanation
Efferent arteriolar constriction raises glomerular capillary hydrostatic pressure, which tends to increase (or preserve) GFR, while simultaneously reducing downstream renal plasma flow — together raising the filtration fraction. This explains why ACE inhibitors, which dilate the efferent arteriole, can acutely reduce GFR in states of renal artery stenosis where efferent constriction was compensating for reduced perfusion.