Langerhans cell histiocytosis

A 7-year-old boy is brought to the clinic by his mother because of severe left leg pain. He localizes the pain to the middle of his left thigh and rates it as a 9 on a 10-point scale with no radiation. He does karate and his family recently got a cat. He denies weight loss, night sweats, or recent trauma. Temperature is 37.3°C (99.2°F), blood pressure is 127/81 mm Hg, pulse is 76/min, respirations are 14/min. X-ray shows a lytic lesion in the mid-shaft of the left femur. Biopsy of the lesion is performed, and microscopic pathology shows numerous cells with clear nuclei and "coffee-bean" shaped nuclei, as well as areas of eosinophils. Which of the following is the most likely diagnosis?