An 18-year-old woman presents to the emergency department with 2 days of right eye swelling. The patient was in her usual state of good health until 2 days ago, when she noted swelling around her right eye. She presents to the emergency department today because the swelling has increased during the past 24 hours and she now has eye pain. There is no history of fever, trauma, recent bug bites, allergic rhinitis, or recent upper respiratory tract infection symptoms.

In the emergency department the patient is afebrile, yet her right eyelids are swollen and mildly erythematous. She has pain with upward and lateral gaze and some mild limitation of superior and lateral right eye movement. There is no nasal congestion; the rest of her physical examination findings are normal.

Several laboratory examinations are performed. A complete blood cell count reveals the following: hemoglobin, 14.7 g/dL (147 g/L); hematocrit, 42.9% (0.43);...

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