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Question: Rash

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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:

pemsou5_wp • September 5, 2023

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  1. Kelly September 5, 2023 - 10:07 pm Reply

    C) Human herpes virus 6
    The classic presentation of roseola infantum, caused most often by HHV-6, is 3-5 days of high fever in an otherwise well-appearing infant, followed by a macular or maculopapular blanching rash distributed as described that appears as the fever resolves. The peak age group is 7-13 months, and 90% of cases occur in < 2 year olds. Roseola is typically benign and the rash resolves quickly within a few days. Allergy to amoxicillin would present soon after the first dose, and typically with hives +/- other symptoms of anaphylaxis. The non-allergic amoxicillin rash that occurs in up to 10% of children taking amoxicillin typically appears on day 3-10, most often day 5-7. Coxsackie virus is associated with herpangina; the child has no lesions on throat exam. Although unimmunized, the measles vaccine is given at 12-15 months anyways, so most 11 month olds would be unimmunized against measles. Children with measles are not well-appearing, often have the 3C’s of cough, coryza, conjunctivitis, and have a rash that starts on the head and spread cephalocaudally and centrifugally.

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