A 7-year-old boy has complained of pain in both thighs and an intermittent limp for one month. He is pale but afebrile, and there is no joint swelling; both hips have mildly reduced movement. Radiographs of the pelvis are normal. His haemoglobin is 78 g/L (115–155), platelet count 62 × 10^9/L (150–450) and white cell count 19.4 × 10^9/L (5.0–15.0); the blood film contains numerous lymphoblasts. Which one of the following is the most likely diagnosis?