Differential Diagnosis for a 14-year-old with Left Knee Pain and Swelling

Single Most Likely Diagnosis

• Juvenile Idiopathic Arthritis (JIA): This condition is a common cause of knee pain and swelling in adolescents without a history of trauma. The positive ANA (Antinuclear Antibody) blood test supports this diagnosis, as a subset of JIA patients can be ANA positive.

Other Likely Diagnoses

• Osgood-Schlatter Disease: A common cause of knee pain in adolescents, especially during growth spurts. It typically affects the tibial tubercle but can cause swelling and pain around the knee.
• Septic Arthritis: Although less likely without a clear source of infection, septic arthritis is a significant concern in any patient with a swollen, painful joint, especially if there's systemic illness or fever.
• Osteochondritis Dissecans: A condition where a piece of cartilage and bone in the joint becomes damaged, leading to pain and swelling. It can occur without trauma, especially in the knee.

Do Not Miss Diagnoses

• Lymphoma or Leukemia: Although rare, these conditions can present with joint pain and swelling. A thorough examination and further testing are crucial if suspected.
• Neuroblastoma: A rare pediatric cancer that can metastasize to bones, causing pain and swelling.
• Slipped Capital Femoral Epiphysis (SCFE): More common in the hip but can refer pain to the knee. It's a hip condition that requires urgent attention to prevent long-term damage.

Rare Diagnoses

• Kawasaki Disease: An acute febrile illness that can cause joint pain and swelling, among other symptoms. It's rare but important to consider due to its potential for cardiac complications.
• Hemophilia: Patients with hemophilia can experience spontaneous joint bleeds, leading to pain and swelling without trauma.
• Sickle Cell Disease: Patients with sickle cell disease can have vaso-occlusive crises that lead to joint pain and swelling due to bone infarcts.

The best treatment plan would depend on the confirmed diagnosis but generally involves a combination of rest, physical therapy, pain management, and in some cases, surgical intervention or disease-modifying antirheumatic drugs (DMARDs) for conditions like JIA.