A high suspicion for malignancies needs to be maintained in order to prevent a delayed diagnosis.
The little girl is fussy. She has been eating and drinking well. She has no upper respiratory, gastrointestinal, or urinary symptoms. There is no history of skin rash, insect, or tick bite. No history of recent travel or sick contacts.
Initial physical exam
The child is alert, oriented, well nourished, but not very cooperative with the examination. She seems to be comfortable in mother's lap. She is afebrile on presentation, with a temperature of 97° F. Her ears and throat appear normal, and conjunctiva is white. Chest is clear to auscultation and her heart sounds are normal. Abdominal palpation is difficult due to her age-appropriate resistance and guarding. No gross abnormality is detected, however. Your patient has no skin rash and no lymphadenopathy. Neurologic exam is nonfocal.
She becomes apprehensive during the examination of the right leg and seems to have mild discomfort on palpation of the right distal thigh. The right leg appears normal with no erythema, swelling, or warmth around the right knee. On careful examination it seems it is more of a periarticular discomfort. When distracted, she exhibits a full range of movement in the knee joint. There is no evidence of injury around the knee. Her right hip, ankle joints, and all other joints are normal. Back and spine exam is unremarkable.
'A localized problem?'
Based on the history and physical exam, you suspect a localized problem around her right knee. In view of the acute onset of symptoms, there could be an infectious etiology. Septic arthritis seems less likely as she moves her knee well when distracted. Toxic synovitis seems less likely, in view of the location of the discomfort and absence of any upper respiratory symptoms prior to the onset of the patient's current symptoms. You remember that referred pain from the hip can cause knee pain, so hip pathology remains in your differential. Osteomyelitis of the femur is possible because of fever and localized bone tenderness.
You order a complete blood count and find a white cell count of 16.1 x 103/μL (normal range for 6- to 23-month-olds: 6.0 to 17.5 x 103/μL) with 49% segmented neutrophils, no bands, and 49% lymphocytes (normal ranges for 12- to 23-month-olds: 13% to 33%, 0% to 4%, and 46% to 76%, respectively). She has moderate normocytic anemia with hemoglobin of 8.9 g/dL and mild thrombocytosis with a platelet count of 798 x 103/μL (normal range for 6- to 23-month-olds: 10.5 to 14.0 g/dL and 150 to 450 x 103/μL). The erythrocyte sedimentation rate (ESR) is elevated at 93 mm/hour (normal for females 1 to 17 years: 4 to 30 mm/hour). C-reactive protein (CRP) is elevated at 4.3 mg/dL (normal: 0 to 1.0 mg/dL). Serum electrolytes including calcium and glucose are normal. Creatinine and liver enzymes are within normal limits.