Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as sports. The chronic nature of the pain and its exacerbation with sports activities make PFPS a highly plausible diagnosis.

Osteoarthritis (OA): Although more common in older adults, OA can occur in younger individuals, especially those with a history of sports injuries or excessive wear and tear on the joints. Years of knee pain that worsens with sports could indicate early-onset OA.
Meniscal Tears: These are common in individuals who participate in sports, especially those that involve twisting motions. Chronic knee pain that worsens with activity could be due to an untreated meniscal tear.
Ligament Sprains: Repeated stress or a significant injury to the knee ligaments (such as the ACL or MCL) during sports can lead to chronic pain and instability, especially if not properly rehabilitated.

Osteonecrosis: Although less common, osteonecrosis (avascular necrosis) of the knee can occur, especially in individuals with a history of steroid use, excessive alcohol consumption, or significant trauma. It's crucial to diagnose early to prevent collapse of the bone.
Infection (Septic Arthritis): While acute infections are more readily identified, a chronic low-grade infection could present with long-standing pain and might be overlooked. This diagnosis is critical to identify promptly to avoid joint destruction.
Tumor: Both benign and malignant tumors can cause chronic knee pain. Although rare, missing a diagnosis of a tumor could have severe consequences.

Osgood-Schlatter Disease: Typically seen in adolescents, this condition involves pain and swelling below the knee due to inflammation of the patellar tendon. It's less likely in adults but could be considered in younger patients.
Sinding-Larsen-Johansson Syndrome: Another condition more common in adolescents, it involves inflammation of the patellar tendon at its insertion on the tibia. Like Osgood-Schlatter, it's less common in adults but could be a rare cause of chronic knee pain.
Complex Regional Pain Syndrome (CRPS): A rare condition characterized by chronic pain, inflammation, and hypersensitivity, usually following an injury. It's an unlikely but possible cause of long-standing knee pain.