What is the most likely diagnosis for an active 13-year-old boy with anterior knee pain?

The most likely diagnosis for an active 13-year-old boy with anterior knee pain is Osgood-Schlatter disease. This condition is common in adolescents who are involved in sports and physical activity, which aligns with the boy's active lifestyle. Osgood-Schlatter disease is characterized by inflammation of the patellar tendon insertion on the tibial tuberosity, often resulting from repetitive stress or overuse during growth spurts.

Typically, the pain is located just below the kneecap (patella) and may be aggravated by activities that involve running, jumping, or kneeling. The prominence of the tibial tuberosity can often be noted, especially after physical activity.

While other conditions, such as patellar tendinitis and patellofemoral syndrome, are also possible in an active youth, they usually present with slightly different symptoms or demographic considerations. An ACL tear is less common in isolated cases in this age group without a specific traumatic or pivoting injury. Therefore, based on the typical presentation, demographic factors, and activity level of the patient, Osgood-Schlatter disease is the most fitting diagnosis.