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Parents bring in their 11 month old baby for a new-onset rash. The baby is unimmunized by parent choice. The baby had fever for the last 3 days to a maximum of 104 F. Although he had fever, he was well-appearing and still eating normally and playful. But since the fever was persistent, they took him to a clinic yesterday and he was diagnosed with a throat infection and started on amoxicillin. He has received 2 doses. Today he awoke with a fine morbilliform blanching pink rash that started on the neck and trunk, then has spread to the face and extremities. It does not seem itchy and it does not involve the mucosa. The only other medication he has had was acetaminophen yesterday at 5pm. On exam, he is nontoxic and interactive. Vital signs: temp 98.9 F, HR 132, RR 28, O2 sat 99% on room air. ENT, chest, and abdomen exams are unremarkable. 

The likely cause of his rash is: