A 14-year-old girl is brought to the physician by her mother for evaluation of several bruises on her lower extremities. She has had these bruises for about 6 weeks, and the mother is concerned that she might be bullied at school. The patient has had increasing fatigue and paleness over the past several days. She has a history of recurrent generalized tonic-clonic seizures treated with carbamazepine. She appears pale and ill. Her temperature is 37.8°C (100.1°F), pulse is 115/min, and blood pressure is 100/60 mm Hg. The lungs are clear to auscultation. Examination shows a soft, nontender abdomen with no organomegaly. There are several subcutaneous purple spots on her legs bilaterally. Her hemoglobin concentration is 8.4 g/dL, leukocyte count is 2,600/mm3, platelet count is 18,000/mm3, and reticulocyte count is 0.3%. Serum electrolyte concentrations are within normal limits. Which of the following is the most likely underlying cause of this patient's symptoms?

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