A mother brings her 3 year-old son to the urgent care clinic. She reports that since this morning, her boy has developed a high fever, joint pains, itching, ankle swelling and erythematous plaques. He is otherwise playful and active. He was prescribed cefaclor 12 days ago by his pediatrician for an ear infection. She stopped cefaclor 2 days ago but wonders why the rash is not going away. On physical examination, the patient has a maculopapular rash on his trunk. Pharyngeal examination is normal. There are no mucosal lesions. He has submandibular lymphadenopathy. Heart and lung examinations are normal. Of the following, which is the most likely diagnosis?
