This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Creatinine clearance is used to assess: \nSelect and explain. \nA. Glomerular function, B. Afferent loop pressure, C. Tubular function, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: An estimate of creatinine clearance is based on the serum creatinine level and is used to measure GFR. Creatinine is a break-down product of creatine phosphate, which is found in muscle. It is freely filtered by the glomerulus, but also actively secreted by the peritubular capillaries in very small amounts such that creatinine clearance overestimates actual GFR by 10-20%. This margin of error is acceptable, considering the ease with which creatinine clearance is measured.', 'role': 'assistant'}]