Aplastic anemia

A 15-month-old boy is brought in to the pediatrician's office for a regular check-up. His mother reports that he had an upper respiratory infection about 2 months ago, but that he has been otherwise healthy since his last visit. Past medical history is non-contributory. Routine screening complete blood count shows a hemoglobin level 6.8g/dL, 5000 WBCs/µL, and 400,000 platelets/µL. A follow-up complete blood count 2 weeks shows the same results, in addition to a reticulocyte count of 0.5%. At this time, mean cell volume is 95fL. Which of the following is the most likely diagnosis in this patient?