Question

A $7$-year-old male child presents to the outpatient clinic accompanied by his mother due to an altered gait that began approximately $8$ weeks ago. The mother describes a progressive limp that worsens towards the end of the day. The child occasionally mentions mild aching in his left groin and distal anterior thigh. He has not had any recent infections, and there is no history of preceding trauma. Vital signs show a temperature of $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), pulse of $84/\text{min}$, and blood pressure of $102/64\text{ mm Hg}$. On physical examination, the patient has a noticeable Trendelenburg gait. Inspection reveals $1\text{ cm}$ of muscle atrophy in the left quadriceps compared to the right. When the patient is supine, the left hip exhibits limited abduction and restricted internal rotation. Examination of the left knee reveals no swelling, erythema, or pain with movement. Laboratory analysis demonstrates a leukocyte count of $7,200/\text{mm}^3$, an erythrocyte sedimentation rate of $10\text{ mm/h}$, and a C-reactive protein of $2.0\text{ mg/L}$. Which of the following is the most appropriate initial step in the evaluation of this patient?