Hello Dr. My son just turned 4 years old and has recently began standing at the toilet to urinate v/s sitting on the toilet. Two times in the past couple of weeks there has been dark particles come out with his urine. I'm keeping an eye on it, hoping he's just catching some dirt off the potty but that's hard for me to believe since it happened once at home where I know the toilet is clean. He's a very picky eater and likes to drink fluids all the time, so his diet is terrible. How concerned should I be?