Osgood-Schlatter disease: This condition is characterized by knee pain in adolescents, particularly during growth spurts, and is often associated with inflammation of the patellar tendon at its insertion on the tibial tubercle. The presentation of "Osgood clatter" likely refers to Osgood-Schlatter disease, given the context of knee pain in a teenager.

Patellofemoral pain syndrome: This condition is common in adolescents and is characterized by pain around or behind the patella, often related to activities that put stress on the knee, such as jumping, running, or climbing stairs.
Medial collateral ligament (MCL) sprain or strain: Although more common in athletes, MCL injuries can occur in teenagers and present with knee pain, especially on the medial aspect.
Meniscal injuries: Tears in the meniscus can cause knee pain, locking, or catching sensations, and are possible in teenagers, especially those involved in sports.

Septic arthritis: Although rare, septic arthritis is a medical emergency that requires prompt diagnosis and treatment to prevent serious complications, including joint destruction and systemic infection.
Osteosarcoma or other bone tumors: While rare, bone tumors can present with knee pain and must be considered, especially if the pain is persistent, severe, or accompanied by other systemic symptoms like weight loss or fever.
Slipped capital femoral epiphysis (SCFE): This condition, more common in obese adolescents, involves the femoral head slipping off the femoral neck and can present with knee pain referred from the hip.

Kohler's disease: A rare condition involving avascular necrosis of the navicular bone, which can cause knee pain due to referred pain from the foot.
Scheuermann's disease: A condition affecting the spine that can cause knee pain due to altered biomechanics and posture.
Chronic recurrent multifocal osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone inflammation, which can cause pain in various parts of the body, including the knees.