Review Question - QID 216615

A 5-year-old boy presents to his primary care physician for recurrent colds, fatigue, and fussiness. Over the past several weeks, the child has been more fatigued and his parents state that “he always seems to be sick.” They state that sometimes he complains about his bones hurting and they note that he is less playful. He is up to date on his vaccinations. His temperature is 102°F (38.9°C), blood pressure is 92/60 mmHg, pulse is 115/min, respirations are 23/min, and oxygen saturation is 99% on room air. Physical exam is notable for diffuse non-tender lymphadenopathy. Abdominal exam reveals hepatosplenomegaly. Laboratory studies are ordered as seen below.

Hemoglobin: 8.0 g/dL
Hematocrit: 23%
Leukocyte count: 27,500/mm^3 with 35% lymphoblasts
Platelet count: 49,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 101 mEq/L
K+: 4.0 Eq/L
HCO3-: 24 mEq/L
BUN: 16 mg/dL
Glucose: 100 mg/dL
Creatinine: 0.7 mg/dL

Which of the following is the most likely diagnosis?