AKT-0585
A 6-year-old girl presents with a rash and joint pain. The rash has developed over the past few days, with the joint pain starting today. She also reports intermittent abdominal pain that has been occurring since before the rash appeared. On examination, she is afebrile with a blood pressure of 110/70 mmHg. There is a symmetrical purpuric rash over the extensor surfaces of her arms and legs and over her buttocks, while her trunk is unaffected. The child complains of pain in her knees and ankles, which appear slightly swollen. Her abdomen is soft with mild periumbilical tenderness on palpation. Which test would be most helpful in guiding further management of this patient?