Review Question - QID 213592

A 5-year-old boy is brought into the pediatrician by his mother for fatigue. The mother reports that the patient used to run outside with his friends, but now she notices he has to stop and rest frequently. She states that he is a “picky eater," and eats mostly chicken and pasta. The mother also notes that the patient seems to have many bruises. The patient started daycare recently, and his teacher says that he gets along well with his classmates. The patient is in the 5th percentile for height and 45th percentile for weight. On physical examination, the patient has conjunctival pallor, bilateral hypoplastic radii, and deformed thumbs. A few scattered ecchymoses are found as well as a hyperpigmented skin lesion, one of which is shown in Figure A. Labs are drawn, as reported below:

Hemoglobin: 8 g/dL
Leukocyte count: 4,000 cells/mm^3 with normal differential
Platelet count: 78,000/mm^3
Reticulocytes: 0.5%

Which of the following is the most likely cause of the patient’s presentation?