A 13-year-old presents with left thigh and knee pain without trauma; what is the most likely diagnosis?

The most likely diagnosis for a 13-year-old presenting with left thigh and knee pain without trauma is slipped capital femoral epiphysis. This condition typically occurs in adolescents during periods of rapid growth, usually between the ages of 10 and 16. The displacement of the femoral head due to slippage at the growth plate can cause pain in the hip that can radiate to the thigh and knee.

In this age group, it is crucial to consider hormonal changes and growth spurts as risk factors. Symptoms may include limited range of motion and joint stiffness, and the child may exhibit a preference for external rotation of the affected leg.

Identifying slipped capital femoral epiphysis early is key to management, as long-term complications can arise if the condition is left untreated. While conditions such as Osgood-Schlatter disease and Legg-Calve-Perthes disease could also present in this age group, the specific presentation of knee and thigh pain without a history of trauma aligns more closely with slipped capital femoral epiphysis. Septic arthritis, while a possibility, typically presents with severe pain, fever, and signs of infection, which are less likely in a case without trauma or systemic symptoms.