Approach to peripheral lymphadenopathy (pediatrics) Quiz: Ace Your Exams

A previously healthy 4-year-old boy is brought to the pediatrician for evaluation of low-grade fevers, weight loss, and refusal to bear weight on the left leg due to pain. He has consistently been in the 50^th percentile for weight, but parents have noticed a 5-pound weight loss over the past month. The patient has not had upper respiratory or gastrointestinal symptoms. They have not traveled and there is no family history of autoimmune disease. Temperature is 38°C (100.4°F), pulse is 140/min, respirations are 24/min, blood pressure is 91/50 mmHg, and oxygen saturation is 99% on room air. On physical examination, the patient appears pale. Ecchymosis is seen over the abdomen and extremities. There are multiple non-tender, firm, rubbery matted lymph nodes palpated in the cervical, axillary, and inguinal regions. There is a soft 2/6 systolic murmur heard best at the left sternal border. There is hepatosplenomegaly. Movement of the left leg causes pain. Which of the following tests will confirm the most likely diagnosis?