Kidney Stones from Med??

My daughter is 17.  She has ulcerative colitis, which is not under control at this time.  She was diagnosed at age 14.  She began the cycle of asacol, which she still takes, steriods, and numerous enemas & foams.  She was put on a 6 mp drug 2 years ago for 8 months, and she started having kidney stones, losing some hair, major fatigue, and the rectal bleeding still did not get better.  She eventually came off of the 6mp, at my insistence, and had a great 18 mo. period w/ periodic relapses.  At the present, she started bleeding again in March, was put on prednisone(which is real hard for 17 yr old w/ weight gain who was normally size 4), and has been on imuran since March.  She had a kidney stone which required cysto. & hospitalization in June.  The urologist today said her 48 urine came back with low output and it was a xanthine stone.  He, himself, said this was rare stone type and was going to call the lytho-lync lab to get more info and is supposed to call me Tues.  I totally think these kidney stones are related to imuran.  I am worried about her taking this drug at such a young age.  Anybody have suggestions or have had any type of the same side effects.  They did a colonoscopy recently and her colitis has improved and the inflammation is at the bottom of the rectum.  I'm wondering if such a powerful drug is even necessary since her bleeding has still not gone away and she has taken imuran since March?