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 climbing stairs, squatting, or jumping. The absence of systemic symptoms and the unilateral nature of the pain make PFPS a strong candidate.

Meniscal Tear: A tear in the meniscus, which is the cartilage that cushions the knee joint, can cause pain and swelling, especially if the tear is in the anterior portion of the meniscus.
Ligament Sprain: Sprains to the ligaments around the knee, such as the medial or lateral collateral ligaments, can cause localized pain and swelling.
Prepatellar Bursitis: Inflammation of the prepatellar bursa, which is located in front of the patella, can cause swelling and pain in the anterior knee.
Osgood-Schlatter Disease: Although more common in adolescents, this condition, which affects the tibial tubercle, can sometimes be seen in adults and causes anterior knee pain.

Septic Arthritis: Although the patient does not have systemic symptoms, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with localized pain and swelling.
Osteonecrosis: Avascular necrosis of the bone around the knee can cause pain and swelling, and it is crucial to diagnose early to prevent further bone damage.
Tumor: Both benign and malignant tumors can cause localized pain and swelling in the knee. Early diagnosis is critical for effective treatment.

Plica Syndrome: This condition involves inflammation of the synovial folds within the knee joint, which can cause anterior knee pain.
Hoffa's Syndrome: Inflammation of the infrapatellar fat pad can cause anterior knee pain and is a less common cause of knee pain.
Sinding-Larsen-Johansson Syndrome: Similar to Osgood-Schlatter disease but affects the lower pole of the patella, causing anterior knee pain.