Patellofemoral Pain Syndrome (PFPS): This condition is the most common cause of knee pain in young patients, especially those who are active in sports. It is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as running, jumping, or climbing stairs. The pain can be exacerbated by sitting for long periods or going up and down stairs.

Ligament Sprains: Injuries to the ligaments surrounding the knee, such as the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), or lateral collateral ligament (LCL), are common in young, active individuals. These injuries often result from sudden stops, changes in direction, or direct blows to the knee.
Meniscal Tears: Tears in the cartilage of the knee joint can cause pain, swelling, and mechanical symptoms like locking or catching. These tears are frequent in young athletes, particularly those involved in contact sports or sports that involve twisting movements.
Osgood-Schlatter Disease: This condition affects the tibial tubercle, where the patellar tendon attaches to the shinbone. It is characterized by pain, swelling, and a lump below the knee and is common in adolescents during growth spurts.
Osteochondritis Dissecans: A joint condition in which a piece of cartilage, along with a thin layer of the bone beneath it, comes loose from the end of a bone because of inadequate blood supply. It can lead to pain and swelling in the affected joint.

Septic Arthritis: Although less common, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It is characterized by severe pain, swelling, redness, and warmth of the joint, often accompanied by systemic symptoms like fever.
Osteomyelitis: An infection of the bone that can cause pain, swelling, and redness over the affected bone. It is a serious condition that requires early diagnosis and treatment to prevent long-term damage.
Tumor (e.g., Osteosarcoma): Although rare, bone tumors can cause knee pain in young patients. Early detection is crucial for effective treatment and prognosis.

Sinding-Larsen-Johansson Syndrome: Similar to Osgood-Schlatter disease but affects the patellar tendon at its attachment to the patella. It is less common and typically seen in younger athletes.
Discoid Meniscus: A rare congenital anomaly of the meniscus that can cause knee pain and mechanical symptoms. It is often diagnosed incidentally during arthroscopy for other conditions.
Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone inflammation and pain, often affecting the metaphyses of long bones. It can mimic other conditions and requires a high index of suspicion for diagnosis.