Obstruction may be from a urinary stone, a tumor, a blood clot, inflammatory narrowing or external pressure from any cause, including an abnormally placed artery to the kidney
. One of the most common causes is enlargement of the prostate
gland. Obstruction in
one ureter, as from a stone, will affect only the kidney on that side.
I would suggest a referral to a urologist for further evaluation of the prostate to ensure it isn't causing the hydronephrosis.