An 18-year-old boy comes to the ED with a 3-day history of a rash. Initially noted on both hands, the discrete, flat, circumscribed red lesions now are present on his arms and feet and are mildly itchy. He reports feeling tired and had experienced headache and subjective fever 2 days ago. His right knee is mildly swollen, causing him to limp. He has not noted any redness around the joint but complains of vague pain. He denies dysuria, back pain, abdominal pain, sore throat, or genital sores. He is sexually active but denies any history of sexually transmitted diseases, illicit drug use, or urinary tract infection.

Physical examination reveals a healthy-looking adolescent who has normal vital signs. Positive findings include injected conjunctivae with no exudates or photophobia. His right knee has a mild effusion but no tenderness or redness. A faint macular, petechial rash is noted on both his palms...

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