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: Surgery
What is the normal level of PSA (prostate specific antigen) in males? 
Select and explain. 
A. < 4 ng/ml, B. 4-10 ng/ml, C. >10 ng/ml, D. PSA is not produced by normal males
A: < 4 ng/ml
Answer: A: < 4 ng/ml 
Explanation: Normal PSA level is less than 4 ng/ml. Prostate-specific antigen (PSA) is an androgen regulated serine protease produced by the prostate epithelium. It is normally present in low concentrations in the blood of all adult males. PSA levels may be elevated in the blood of men with benign prostate conditions such as prostatitis and benign prostatic hyperplasia, as well as in men with prostate cancer. It is useful in evaluating the effectiveness of prostate cancer treatment and monitoring for recurrence after therapy.