The dark urine could be due to less water intake and may be due to intake of B complex vitamins.
Other reason for dark urine is jaundice
but for that you must have some symptoms like nausea,vomiting,loss of appetite etc.
Dark urine and slightly enlarged prostate
do not indicate any prostate cancer
If you have apprehension regarding prostate cancer,consult a surgeon and get your PSA level checked up.
Your ultra sound
doesn't give any indication of prostate cancer.