Which criterion would most likely prompt further investigation for septic arthritis in a child?

The refusal to bear weight is a significant indicator that would prompt further investigation for septic arthritis in a child. When a child exhibits this symptom, it often suggests severe pain or discomfort in the affected joint, which can be consistent with an infectious process such as septic arthritis. This symptom, combined with other clinical signs, highlights the need for further evaluation, including imaging studies and possibly joint aspiration, to rule out infection.

In clinical practice, weight-bearing is a key factor in assessing orthopedic conditions in children, and sudden refusal to bear weight is often concerning for underlying pathology. In the context of septic arthritis, where early detection and intervention are critical to prevent joint damage and systemic complications, a child’s inability or unwillingness to bear weight merits immediate attention.

Other parameters, such as a WBC count under 12,000, a fever less than 38 degrees Celsius, or an ESR less than 40, while informative in assessing the child's overall health, are less specific indicators for septic arthritis. These values may not alone indicate the presence of infection and can sometimes fall within normal ranges even in cases of significant underlying pathology. Therefore, the refusal to bear weight stands out as a clinical red flag warranting further investigation.