Osgood-Schlatter disease: This condition is a common cause of knee pain in adolescents, particularly during periods of rapid growth. 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. The pain typically worsens with activity and improves with rest, matching the patient's symptoms.

Patellofemoral pain syndrome: This condition is also common in adolescents and young adults, especially those who are active in sports. It is characterized by pain around or behind the patella, often related to activities that put stress on the knee, such as running, jumping, or climbing stairs.
Medial collateral ligament (MCL) sprain or strain: Although the patient does not report a specific injury, MCL sprains or strains can occur from overuse or minor trauma and may not always have a clear incident. Pain is typically localized to the medial aspect of the knee.
Meniscal tear: Meniscal tears can occur in adolescents, especially those involved in sports. They may present with pain, especially with twisting or bending movements, and can be associated with locking or catching sensations in the knee.

Osteosarcoma or other bone tumors: Although rare, bone tumors such as osteosarcoma can present with progressive pain that worsens over time. It is crucial to consider these diagnoses to ensure timely intervention if present.
Septic arthritis: This is an infection within the joint space, which can present with pain, swelling, and decreased range of motion. It is a medical emergency requiring prompt diagnosis and treatment.
Slipped capital femoral epiphysis (SCFE): This condition occurs when the ball at the upper end of the femur (thigh bone) slips out of place, and is more common in obese adolescents. It can present with knee pain, as the pain is often referred from the hip.

Chronic recurrent multifocal osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone inflammation, which can cause pain and swelling in various parts of the body, including the knees.
Kohler's disease: A rare condition that affects the tibial tubercle, causing pain and swelling just below the knee, similar to Osgood-Schlatter disease but typically affecting younger children.
Sinding-Larsen-Johansson syndrome: Similar to Osgood-Schlatter disease but affects the lower pole of the patella, causing pain and swelling at the upper end of the patellar tendon.