Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or under the patella, often associated with activities that put stress on the knee, such as climbing stairs, squatting, or repetitive motion. The pain is typically exacerbated by activities that increase pressure between the patella and the femoral condyles.

Chondromalacia Patellae: A condition where the cartilage on the underside of the patella deteriorates and becomes soft, leading to knee pain and tenderness. It is often associated with overuse or repetitive motion.
Patellar Tendinopathy: Inflammation of the patellar tendon, which connects the patella to the tibia. This condition is common in athletes who jump frequently, such as basketball players, but can also occur in individuals who engage in activities that involve repetitive knee bending.
Iliotibial (IT) Band Syndrome: Although more commonly associated with lateral knee pain, IT band syndrome can sometimes cause pain under the patella, especially if the IT band is tight and pulls on the patella.

Osteonecrosis of the Knee: A condition that occurs when there is a loss of blood to the bone, leading to tiny breaks in the bone and the bone's eventual collapse. It can cause severe knee pain and is more common in older adults or those with certain medical conditions.
Infection (Septic Arthritis): Although less common, an infection within the knee joint can cause severe pain, swelling, and warmth. It is a medical emergency that requires prompt treatment.
Tumor: Both benign and malignant tumors can occur in the knee, causing pain and other symptoms. While rare, it is crucial not to miss these diagnoses.

Sinding-Larsen-Johansson Syndrome: A rare condition that affects the patellar tendon at its attachment to the tibia, causing knee pain and swelling. It is more common in adolescents.
Osgood-Schlatter Disease: Although it typically causes pain and swelling below the knee, in rare cases, it can refer pain to the area under the patella. It is a condition that affects the tibial tubercle.
Bursitis: Inflammation of one of the bursae around the knee can cause localized pain and swelling. While not typically under the patella, certain bursae (like the deep infrapatellar bursa) could potentially cause pain in this area.