Osgood-Schlatter Disease: This condition is a common cause of knee pain in adolescents, particularly during growth spurts. It is characterized by inflammation of the patellar tendon at its insertion on the tibial tubercle, leading to pain and swelling just below the knee.

Patellofemoral Pain Syndrome: This condition is also 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, a sprain or strain of the MCL can occur in any active adolescent, leading to medial knee pain.
Meniscal Injury: Tears in the meniscus can occur in adolescents, especially those involved in sports, leading to knee pain, locking, or catching sensations.

Septic Arthritis: Although less common, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It presents with severe knee pain, swelling, redness, and warmth, often accompanied by systemic symptoms like fever.
Osteosarcoma or Other Bone Tumors: While rare, bone tumors can present with persistent, unexplained knee pain in adolescents. Early diagnosis is crucial for effective treatment.
Slipped Capital Femoral Epiphysis (SCFE): This condition, more common in obese adolescents but can occur in those with normal weight, involves the femoral head slipping off the femoral neck, leading to hip and potentially referred knee pain.

Kohler Disease: A rare condition involving avascular necrosis of the navicular bone in the foot, but can sometimes refer pain to the knee.
Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare inflammatory bone condition that can cause recurrent episodes of bone pain, including in the knee area.
Juvenile Idiopathic Arthritis (JIA): While not exclusively rare, JIA can present with knee pain among other symptoms, and its diagnosis requires consideration of systemic and autoimmune factors.