Rheumatoid Arthritis (RA): This is the most likely diagnosis given the question's context. RA is a chronic autoimmune disorder characterized by symmetric polyarthritis, often affecting hands, feet, wrists, elbows, knees, ankles, and shoulders. It leads to inflammation, pain, swelling, and potentially severe joint damage. The diagnosis is typically made based on a combination of clinical presentation, laboratory tests (including rheumatoid factor and anti-citrullinated protein antibody), and imaging studies.

Osteoarthritis (OA): Although OA is more commonly associated with wear and tear of the joints and is less inflammatory than RA, it can sometimes present with significant inflammation and may be considered in the differential, especially in older patients.
Psoriatic Arthritis (PsA): This condition occurs in some patients with psoriasis, characterized by joint inflammation, enthesitis, and dactylitis. The presence of psoriatic skin lesions or nail changes can help differentiate it from RA.
Lupus: Systemic Lupus Erythematosus (SLE) is an autoimmune disease that can affect multiple systems, including the joints, skin, kidneys, and others. Joint pain and arthritis are common manifestations, and the diagnosis should be considered, especially in patients with other systemic symptoms.

Septic Arthritis: This is an infection within a joint, usually caused by bacteria. It is a medical emergency requiring prompt diagnosis and treatment to prevent joint destruction. Fever, significant pain, and swelling of a single joint are typical presentations.
Gout: An acute inflammatory arthritis caused by deposition of urate crystals within the joints. It often affects a single joint, especially the big toe, and can be extremely painful. Missing this diagnosis could lead to unnecessary treatment with immunosuppressants and failure to address the underlying metabolic issue.
Lyme Arthritis: Caused by the bacterium Borrelia burgdorferi, transmitted by tick bites. It can cause arthritis, particularly of the knee, and is a significant consideration in endemic areas.

Relapsing Polychondritis: A rare autoimmune disorder characterized by recurrent episodes of cartilaginous inflammation, which can affect the ears, nose, trachea, and joints.
Adult-Onset Still's Disease: A rare inflammatory disorder that can present with high fevers, salmon-colored rash, and arthritis, among other symptoms.
Sjögren's Syndrome: An autoimmune disorder that primarily affects the glands that produce tears and saliva but can also cause arthritis and other systemic symptoms.