A 12-month-old African-American boy presents with a 2-day history of irritability and refusal to bear weight. His 16-year-old mother denies any recent history of fever, vomiting, diarrhea, rash, or trauma. She does admit to giving her son a daily medicine that the pediatrician said would 'keep him from getting sick'. Family history is significant for a maternal uncle who had a stroke and died when he was 35 years of age. On exam, vital signs are stable. Patient is a well-developed, well-nourished toddler who appears irritable, but consolable. He is in no apparent distress; however, he does refuse to bear weight or play. The only significant findings on his exam are swollen hands and feet. CBC reveals WBC 18,000 mm3, with 40% neutrophils, 30% lymphocytes, and 1% monocytes. Hb is 8 g/dL and platelets are 400,000 mm3.What test would be helpful in making a diagnosis?