Juvenile Idiopathic Arthritis (JIA): This is the most common chronic arthritis in children, often presenting with oligoarthritis (involvement of fewer than five joints). The diagnosis is based on the presence of arthritis in one or more joints for at least 6 weeks in a child under the age of 16, with other causes of arthritis excluded.

Infectious Arthritis: Bacterial, viral, or fungal infections can cause oligoarthritis, especially if the infection is localized to a single joint or a few joints. Recent travel history, exposure to sick contacts, or underlying immunocompromised conditions can increase the likelihood.
Post-Infectious Arthritis: Conditions like reactive arthritis can occur after certain bacterial infections, leading to oligoarthritis. A history of recent gastrointestinal or genitourinary infection may be present.
Lyme Arthritis: Caused by Borrelia burgdorferi, transmitted by tick bites. It often presents with oligoarthritis, particularly affecting the knee, and is more common in regions where Lyme disease is endemic.
Psoriatic Arthritis: Part of the spondyloarthritis group, it can present with oligoarthritis and is often associated with psoriasis, dactylitis, or a family history of psoriasis.

Septic Arthritis: Although less common, it is a medical emergency requiring prompt diagnosis and treatment to prevent joint destruction and potential systemic complications. Risk factors include recent joint surgery, immunocompromised state, or intravenous drug use.
Tuberculous Arthritis: More common in endemic areas or in individuals with a high risk of exposure to tuberculosis. It can present with chronic oligoarthritis and systemic symptoms like weight loss and fever.
Lymphoma: Though rare, certain types of lymphoma can initially present with oligoarthritis, making it crucial to consider in the differential diagnosis, especially if there are systemic symptoms like night sweats, weight loss, or lymphadenopathy.

Sarcoid Arthritis: Part of sarcoidosis, a systemic granulomatous disease that can rarely present with oligoarthritis, often accompanied by other systemic manifestations like lung involvement or skin lesions.
Enteropathic Arthritis: Associated with inflammatory bowel diseases like Crohn's disease or ulcerative colitis, it can present with oligoarthritis, particularly in the lower limbs, and is often accompanied by gastrointestinal symptoms.
Whipple's Disease: A rare systemic bacterial infection that can cause oligoarthritis among other symptoms like diarrhea, weight loss, and abdominal pain. It's crucial to consider in patients with unexplained, chronic symptoms.