The patient is a 2 year old male who presents with bruising and petechiae more prominent on the legs. Laboratory tests reveal a normal hemoglobin and white blood cell count with a platelet count of 15,000/mm3. The most likely diagnosis is immune thrombocytopenic purpura (ITP). ITP is an autoimmune disorder causing decreased platelet counts due to platelet destruction by autoantibodies. It commonly presents after a viral illness with bruising and petechiae, and the patient's presentation and laboratory results are consistent with ITP.