This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
What would tend to decrease GFR by more than 10% in a normal kidney? 
Select and explain. 
A. Decrease in renal aerial pressure from 100 to 85 mm Hg, B. 50% decrease in afferent aeriolar resistance, C. 50% decrease in efferent aeriolar resistance, D. 50% increase in the glomerular capillary filtration coefficient
C: 50% decrease in efferent aeriolar resistance
Answer: C: 50% decrease in efferent aeriolar resistance 
Explanation: 50% reduction in efferent aeriolar resistance would cause a large decrease in GFR--greater than 10%. A decrease in renal aery pressure from 100 to 85 mm Hg would cause only a slight decrease in GFR in a normal, autoregulating kidney. A decrease in afferent aeriole resistance, a decrease in plasma colloid osmotic pressure, or an increase in the glomerular capillary filtration coefficient would all tend to increase GF