Henoch-Schonlein purpura Quiz: Ace Your Exams

An 8-year-old boy is brought to the emergency department by his parents for evaluation of a rash that developed over both legs 3 days ago. The patient has also been experiencing intermittent moderate abdominal pain that comes and goes and joint pain that comes and goes and migrates from one joint to another. He has no significant past medical history. Temperature is 37.2°C (99°F), blood pressure is 110/62 mmHg, pulse is 101/min, respiratory rate is 20/min, and oxygen saturation is 98% on room air. Palpable reddish-purple lesions approximately 5mm in diameter and appearing in clusters are seen over the legs bilaterally (as shown in the image below). There is swelling, tenderness to palpation and pain with range of motion of the of the right knee. Cardiopulmonary exam is within normal limits. The abdomen is soft, nontender and without masses. CBC and coagulation studies are within normal limits. Which of the following additional findings is most likely, given the likely underlying condition?
 
^{Reproduced from wikipedia}