Steven is a 10-year-old white male seeking medical attention because of possible physical abuse observed by his gym teacher. The teacher noticed severe bruising over much of his upper body when his shirt "rode up" during an exercise. Steven emphatically states that he has never been abused by anyone. He denies any accidents and didn't tell anyone about his bruises because he thought they might "get me into trouble." His mother cannot explain the bruises either and did not see them before today because her son does all of his own hygiene and is quite modest about revealing his body even to family members. Steven's physical examination is benign except for multiple, irregular dark purple bruises over most of his torso and lower extremities. His complete blood count is well within normal limits except for a low platelet count of 18,000/mm3. What is Steven's diagnosis?