Differential Diagnosis

The patient's presentation of joint pain, swelling, and stiffness, particularly in the morning, along with a positive serum rheumatoid factor and elevated C-reactive protein, suggests an inflammatory arthritis. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Rheumatoid Arthritis (RA): The patient's symptoms of morning stiffness lasting several hours, joint pain, and swelling, combined with a positive rheumatoid factor and elevated inflammatory markers, are classic for RA. The involvement of small joints of the hands, such as the metacarpophalangeal joints, is also typical.

• Other Likely Diagnoses

  • Psoriatic Arthritis (PsA): Although less likely given the lack of mention of skin or nail changes, PsA can present with similar joint symptoms and elevated inflammatory markers. It often involves the distal interphalangeal joints and can have a variable pattern of joint involvement.
  • Osteoarthritis (OA): While OA is more commonly associated with later age and involves the distal interphalangeal joints and thumbs, it can present in younger individuals, especially with a family history of joint disease. However, morning stiffness lasting hours and significant systemic inflammation are less typical for OA.
  • Systemic Lupus Erythematosus (SLE): SLE can cause arthritis that resembles RA, with small joint involvement and morning stiffness. However, other systemic symptoms such as skin rashes, renal disease, or hematologic abnormalities are usually present.

• Do Not Miss Diagnoses

  • Septic Arthritis: Although the patient's presentation does not strongly suggest septic arthritis (e.g., no mention of fever, single joint involvement, or recent trauma), it is a diagnosis that must be considered due to its potential for rapid joint destruction and serious morbidity.
  • Lyme Arthritis: Caused by Borrelia burgdorferi, Lyme disease can lead to arthritis, particularly of the knee, and is a consideration in endemic areas or with a history of tick exposure.

• Rare Diagnoses

  • Sjögren's Syndrome: An autoimmune disorder that can cause arthritis similar to RA, often accompanied by dry eyes and mouth. It's less common and would typically present with additional symptoms related to exocrine gland dysfunction.
  • Mixed Connective Tissue Disease (MCTD): Overlaps features of SLE, RA, scleroderma, and polymyositis, with a high level of anti-U1 RNP antibodies. It's a rare condition and would likely have a more complex presentation involving multiple systems.