This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary tract, Subject name: Pathology
[{'content': 'Which of the following is not a variant of PSA \nSelect and explain. \nA. PSA density, B. PSA nodularity, C. PSA velocity, D. Ratio of free to bound PSA', 'role': 'user'}, {'content': 'Answer: B \nExplanation: In the past, most doctors considered PSA levels of 4.0 ng/mL and lower as normal. Therefore, if a man had a PSA level above 4.0 ng/mL, doctors would often recommend a prostate biopsy to determine whetherprostate cancer was present.', 'role': 'assistant'}]