Review Question - QID 214213

A 5-year-old-boy presents to the pediatrician's office for petechiae, increased bruising for the past 4 months, and progressive fatigue over the past 6 weeks. Although the patient eats a well-balanced diet and has received all age-appropriate vaccinations, he is at the 3rd percentile for height and 10th percentile for weight for his age. When the pediatrician speaks to him, he answers in 3-word phrases. On physical exam, he is noted to have a short stature, a webbed neck, hypoplastic thumbs, and a fixed split second heart sound. His skin exam demonstrates the finding seen in Figure A on the surface of his inner arm. Laboratory results demonstrate the following:

Hemoglobin: 7.2 g/dL
Mean corpuscular volume: 110/µm³
Platelets: 30,000/mm³
Leukocytes: 3,000/mm³

Which of the following is the most likely diagnosis?