Medial Collateral Ligament (MCL) Sprain: This is a common injury in adolescents, especially during sports activities. The MCL is prone to sprains due to valgus stress, which can occur when playing sports that involve sudden changes in direction or direct blows to the knee.

Anterior Cruciate Ligament (ACL) Sprain: Similar to MCL sprains, ACL injuries are common in adolescents, particularly in sports that involve jumping, landing, and quick turns. Mild inflammation could be an early sign.
Meniscal Tear: Meniscal tears can occur from a sudden twist or direct blow to the knee, leading to inflammation and pain. Adolescents involved in sports are at higher risk.
Patellofemoral Pain Syndrome: This condition, also known as runner's knee, is characterized by pain around or behind the patella, often due to overuse or direct trauma, which could lead to mild inflammation.

Septic Arthritis: Although less common, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with joint pain, swelling, and inflammation, similar to a sports injury, but is typically accompanied by systemic symptoms like fever.
Osteonecrosis (Avascular Necrosis): This condition involves the death of bone tissue due to a lack of blood supply. It can be a complication of trauma or steroid use and requires early detection to prevent long-term damage.
Tumor (e.g., Osteosarcoma): Bone tumors, although rare, can present with pain and swelling in the affected limb. Early diagnosis is crucial for effective treatment.

Osgood-Schlatter Disease: A condition characterized by inflammation of the patellar ligament at the tibial tubercle, leading to a painful lump just below the knee. It's more common in adolescents during growth spurts.
Sinding-Larsen-Johansson Syndrome: Similar to Osgood-Schlatter disease but involves the patellar tendon at its origin, causing inflammation and pain at the lower pole of the patella.
Discoid Meniscus: A rare congenital anomaly of the meniscus that can lead to meniscal tears and symptoms similar to those of a common meniscal injury.