Osgood-Schlatter Disease: This 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 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. It's common in adolescents and young adults.
Medial Collateral Ligament (MCL) Sprain or Strain: Injury to the MCL can occur from a direct blow to the knee or a sudden twisting motion, leading to pain and instability on the inner aspect of the knee.
Meniscal Injuries: Tears in the menisci, which are cartilage structures in the knee joint, can cause pain, locking, or catching sensations in the knee, especially with twisting movements.
Growing Pains: While not exclusively a knee issue, growing pains can manifest as leg pains, including around the knee area, in children and adolescents.

Septic Arthritis: An infection within the joint space, which is a medical emergency requiring prompt antibiotic treatment and possibly surgical drainage to prevent long-term joint damage.
Osteosarcoma or Other Bone Tumors: Although rare, bone cancers can present with knee pain and must be considered, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or fever.
Slipped Capital Femoral Epiphysis (SCFE): A condition where the ball at the upper end of the femur (thigh bone) slips out of place, more common in obese adolescents. It can cause hip pain that may be referred to the knee.

Kohler Disease: A rare condition causing pain and swelling in the navicular bone of the foot but can sometimes refer pain to the knee area.
Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone infection, which can cause pain in various bones, including those around the knee.
Juvenile Idiopathic Arthritis (JIA): While not rare, it's less common than other causes of knee pain in children. It can cause knee pain, swelling, and stiffness, often accompanied by systemic symptoms.