This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiac Output, Subject name: Physiology
[{'content': 'A 60-year old patient underwent renal aery doppler which shows narrowing and turbulence in right renal aery. If radius of the aery is reduced by 1/3rd, resistance to blood flow in right kidney would have increased by: \nSelect and explain. \nA. 3 times, B. 9 times, C. 16 times, D. 81 times', 'role': 'user'}, {'content': 'Answer: D \nExplanation: "Fouh power law" of radius ~ Blood flow varies directly and resistance inversely with the fouh power of the radius (r 4). If the radius is changed by a factor of 3 (reduced by 1/3rd), resistance would change by (34).', 'role': 'assistant'}]