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Even if the prostate has old semen, if there is no enlargement or if there is no growth from prostate around the bladder neck (as in cancer), prostate cannot be the cause for urinary retention.
Hence other causes will have to be investigated such as UTI with inflammation of urethra; stone any where in the ureter, kidney or urethra or at the bladder neck; certain drugs like antidepressants, antiallergics etc; constipation, urethral stricture, nerve diseases as in diabetes, MS, pelvic trauma etc.
I would suggest you get another opinion from a third urologist. An ultrasound scan to rule out stones and a urethroscopy or IVP to rule out stricture may be needed. Definitely further tests should be carried out. Please let me know if there is any thing else and do keep me posted. Take care!