A patient with nephrotic syndrome has reduced plasma oncotic pressure. This affects the Starling forces at the glomerulus. What is the NET effect on GFR?

• A GFR initially increases because reduced plasma oncotic pressure reduces the opposition to glomerular filtration, increasing net filtration pressure ✓
• B GFR decreases because protein loss reduces plasma viscosity, slowing blood flow and thus filtration
• C GFR is unchanged because tubuloglomerular feedback compensates within minutes via macula densa sensing
• D GFR decreases because low oncotic pressure causes sodium retention and thus reduced renal plasma flow

Explanation

Glomerular filtration pressure is determined by: GFR ∝ (Pgc − Pbs) − (πgc − πbs), where Pgc = glomerular capillary hydrostatic pressure, Pbs = Bowman's space hydrostatic pressure, πgc = glomerular capillary oncotic pressure, and πbs ≈ 0 (since normally no protein enters Bowman's space). In nephrotic syndrome, massive proteinuria reduces plasma oncotic pressure (πgc), decreasing the opposition to filtration (the oncotic back-pressure). This increases net filtration pressure and GFR. However, in practice, many nephrotic patients develop mesangial changes, reduced Kf, or volume depletion that may offset this; nevertheless, the direct Starling effect of reduced πgc is to increase GFR. Option D reverses the logic.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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