A previously healthy and fully vaccinated 16-year-old boy presents to the emergency department after 3 days of evolving left lower extremity pain and weakness. Pain of sudden onset localized to the left anterior hip had begun 3 evenings earlier. The morning after onset, he was unable to bear weight. He noticed radiation of pain to his left thigh. The following morning, he awoke with a temperature of 102.2°F (39°C), with continued inability to bear weight and subjective weakness of his left leg, prompting presentation to the emergency department. Radiation to the lumbosacral region, left buttock, and left knee is now present. He has not experienced swelling, redness, or warmth of any joint. His pain is unchanged in intensity relative to onset. He has not taken an analgesic. He denies any preceding trauma and is not active in sports. The patient’s medical history is normal. He takes no medications. He had...

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