A 7-year-old previously healthy black male child presented to the hospital with left knee cellulitis. Two days prior to admission he had been in a fight and had injured his left knee. The knee became progressively swollen, red, and intermittently painful. The patient also complained of headache and abdominal pain without nausea, vomiting, or diarrhea. He had no history of fever. Medical history was noncontributory.
On physical examination, the temperature was found to be 39°C, and there was a 5.5×5-cm erythematous, indurated, warm lesion on the medial aspect of the left knee (Fig 1). The knee was not tender to palpation, and had full range of motion. There was no break in the skin integrity and no inguinal adenopathy. The white blood cell count was 18×109/L; platelets, 272×109/L; hematocrit, 0.33; and differential cell counts were as follows: neutrophils, 0.59; bands, 0.07; lymphocytes, 0.16; monocytes, 0.02; and