MSRA-0956

A 6-year-old boy is brought to see his General Practitioner by his father, who is concerned as his son has a 4-day history of abdominal pain and fever. He seems to have lost his appetite and has been waking in the night with night sweats for the last week.
On examination, there is a large palpable abdominal mass and hepatomegaly. A urine dipstick is negative for blood, protein, leukocytes and nitrates.
What is the most likely diagnosis?