A 15-year-old boy presented with a 3-week history of inner left thigh pain provoked by activity and experienced occasionally at rest. He denied nighttime pain, fever, or chills. Laboratory investigation revealed the following normal values: hemoglobin level of 15.6 g/dL (normal range, 13–16 g/dL), platelet count of 240 × 103/µL (normal range, 140– 440 × 103/µL), and total leukocyte count of 7100 cells/µL (normal range, 4500–11 000 cells/µL). The percentage of neutrophils was considered low at 44% (normal range, 54%–62%), and the percentage of eosinophils was slightly high at 3.7% (normal range, 0%–3%). An anteroposterior radiograph of the left hip is shown (Fig 1). Physical therapy was initiated, with no improvement after 2 weeks of therapy. The patient was referred to an orthopedist for further evaluation. On physical examination, the patient endorsed marked left hip pain with hip flexion to 90°, limited internal and external rotation (5° and 15°, respectively), and antalgic gait favoring the left leg. Hip MRI (Fig 2) and further serologic analysis were requested for further evaluation. Although the serologic testing was performed at an outside laboratory, the physician reported positive immunoglobulin-G Lyme titers, normal C-reactive protein level, and normal erythrocyte sedimentation rate. Pelvic CT was requested (Fig 3). The patient was prescribed a course of doxycycline (100 mg twice daily for 28 days), with reported resolution of symptoms 2 weeks after initiation of treatment. Three weeks later, he presented to our department with recurrent left hip pain, which was similar in severity compared with initial presentation. A second MRI of the left hip was performed 4 months after initial presentation (Fig 4).The anteroposterior radiograph of the left hip was normal and demonstrated no evidence of fracture or dislocation (Fig 1). Initial MRI, acquired at an outside institution, demonstrated focal geographic bone marrow edema–like signal intensity centered in the subchondral medial acetabular wall (Fig 2A). Juxtacortical soft-tissue edema was also present, particularly along the ligamentum teres, near the acetabular fovea (Fig 2B). No discrete fracture line was identified. No large hip joint effusion was present. Intravenous contrast material was not administered. CT images were prospectively interpreted as normal. In retrospect, there was a focal ovoid intraarticular lucent focus with sclerotic margins emanating from the medial acetabular wall, near the ligamentum teres, suspicious for a small osteoid osteoma nidus (Fig 3). At repeat MRI 4 months later (Fig 4), there were persistent geographic bone marrow edema–like changes centered in the subchondral medial acetabular wall, which was similar in appearance to the initial MRI findings (Fig 2). Synovial enhancement consistent with synovitis was noted, as well as focal enhancement of the intra-articular nodular focus near the acetabular fovea. No large joint effusion was identified. Given the persistence of geographic bone marrow edema–like changes centered in the subchondral medial acetabular wall, enhancing intra-articular nidus, and left hip synovitis, a diagnosis of intra-articular osteoid osteoma was suggested. Bone scintigraphy was recommended for further assessment. Because fluorine 18–labeled sodium fluoride (18F-NaF) PET/CT has superior spatial resolution and image contrast compared with technetium 99m–labeled methylene diphosphonate (99mTc-MDP) bone scintigraphy, it was the modality of choice to evaluate the intra-articular nidus (Fig 5A, 5B). Approximately 30 minutes after intravenous administration of 4.38 mCi of 18F-NaF , PET/CT imaging of the pelvis was performed. CT images were used for anatomic localization and proton attenuation correction (Fig 5C, 5D). Intense radiotracer uptake was noted at the articular surface of the left acetabulum (Fig 5A, 5B), corresponding to the site of exophytic proliferative new bone formation identified in the attenuation-correction CT images (Fig 5C, 5D) . The focal increased uptake also corresponded to the enhancing intra-articular nidus on the repeat MRI scans (Fig 4A, 4B). These PET/ CT findings confirmed the diagnosis of intra-articular osteoid osteoma. What is your diagnosis?