A 9-YEAR-OLD BOY had a 5-day history of insidious, nontraumatic onset of pain in his right hip and thigh and an associated limp.
On clinical examination, his right hip was held flexed, abducted, and externally rotated. He experienced extreme pain on extension and internal rotation of the hip but little pain on flexion. Mild tenderness was found on palpation of the right lower quadrant. The total white blood cell count was 13.0×109/L; marked lymphocytosis was noted. The sedimentation rate was 36 mm/l h.
A clinical diagnosis of septic arthritis was made, and an ultrasonographic examination of the hip was performed. The hip joint showed no effusion. Anterior to the joint, the psoas muscle appeared enlarged and contained multiple focal hypoechoic areas with internal echoes (Figure 1). More superiorly, within the abdomen, the psoas muscle appeared normal. A computed tomographic scan confirmed the abnormality seen on ultrasonographic examination (Figure