A 10-year-old boy presents with diffuse joint pain and bruising. What condition is most likely given his Down syndrome?

In a 10-year-old boy with Down syndrome presenting with diffuse joint pain and bruising, the most likely condition is leukemia. Children with Down syndrome are at a higher risk for developing certain hematologic conditions, including leukemia, particularly acute lymphoblastic leukemia (ALL).

The symptoms of diffuse joint pain and bruising can be significant in this context, as leukemia often leads to thrombocytopenia (low platelet count), which results in bruising and increased bleeding tendencies. Additionally, the infiltration of leukemia cells can also cause bone and joint pain, making it align with the child's presentation.

While hemophilia can also cause bruising and joint pain due to bleeding disorders, it is less common in this population and typically presents differently. Osteogenesis imperfecta primarily involves issues with bone fragility and fractures rather than the combination of joint pain and bruising seen here. Juvenile idiopathic arthritis, although it can cause joint pain, is less directly associated with the bruising symptom and does not typically occur with the elevated incidence in children with Down syndrome as much as leukemia does.

Therefore, the combination of the child's symptoms and the association of Down syndrome with an increased risk for leukemia supports the conclusion that leukemia is the most likely condition in this scenario.