What Causes Bruising Below The Toes Of Children?

My baby has bruising just below his toes on the dorsal surface of his foot. The bruise has been there for a few weeks now and has not gone away. Last week the entire foot got red and became swollen and he would not allow us to touch the foot. We took him to the ER thinking it may have a fracture but the x-ray was neg. The swelling has gone down however the bruise remains. We did treat him for a bug bite but it was not effective. The pediatrician wants to take blood and a urine sample, what could he be trying to rule out?