A 70-year-old man with benign prostatic hyperplasia develops acute urinary obstruction. His serum creatinine rises from 1.0 to 3.5 mg/dL over 48 hours. A renal ultrasound shows bilateral hydronephrosis. Assuming complete obstruction is relieved, which of the following best describes the expected change in GFR and the underlying Starling force alteration that caused the initial decline?
- A GFR fell because glomerular capillary oncotic pressure rose along the capillary length
- B GFR fell because glomerular capillary hydrostatic pressure fell due to afferent arteriolar constriction
- C GFR fell because Bowman's space hydrostatic pressure rose, reducing net filtration pressure ✓
- D GFR fell because the filtration coefficient (Kf) decreased due to mesangial cell contraction
Correct answer: C. GFR fell because Bowman's space hydrostatic pressure rose, reducing net filtration pressure
Explanation
In bilateral urinary obstruction, increased tubular pressure is transmitted retrograde to Bowman's space, raising Bowman's space hydrostatic pressure (PBC) and directly reducing net filtration pressure. This is the immediate cause of the GFR decline. Distractor B describes prerenal azotemia. Distractor A describes filtration equilibrium in a normally functioning glomerulus. Distractor D describes mesangial contraction reducing Kf, which occurs in some glomerular diseases but not acutely in obstruction.
Reference: Brenner and Rector's The Kidney, 11th ed.
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