Differential Diagnosis

The patient's presentation of pain and swelling of the left knee and ankle, along with conjunctivitis, balanitis, and stomatitis, suggests a condition that affects multiple systems of the body. Here's a categorized differential diagnosis:

• Single most likely diagnosis

  • Reactive Arthritis (ReA): This condition is characterized by the development of arthritis following an infection, often of the gastrointestinal or genitourinary tract. The presence of conjunctivitis, balanitis, and stomatitis, along with joint symptoms, is highly suggestive of Reactive Arthritis, particularly if the patient has recently had a bacterial infection such as Chlamydia or Salmonella.

• Other Likely diagnoses

  • Psoriatic Arthritis: This condition can cause joint pain and swelling, and is often associated with skin and mucous membrane findings such as balanitis and stomatitis. However, the absence of psoriatic skin lesions in the question makes this less likely.
  • Rheumatoid Arthritis: Although it can cause joint pain and swelling, the presence of conjunctivitis, balanitis, and stomatitis is less typical for Rheumatoid Arthritis.
  • Spondyloarthritis: This category of diseases includes conditions like ankylosing spondylitis and enteropathic arthritis, which can present with joint symptoms and extra-articular manifestations like conjunctivitis and skin lesions.

• Do Not Miss

  • Septic Arthritis: Although less likely given the combination of symptoms, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. The presence of fever, significant joint pain, and swelling, especially if asymmetric, should raise suspicion.
  • Lyme Disease: In endemic areas, Lyme disease can cause arthritis and other systemic symptoms. The presence of a rash (erythema migrans) and a history of tick exposure would support this diagnosis.

• Rare diagnoses

  • Behçet's Disease: A rare condition characterized by recurrent oral ulcers, genital ulcers, and uveitis, which can also cause arthritis. The combination of stomatitis, balanitis, and joint symptoms could suggest this diagnosis, but it is less common.
  • Whipple's Disease: A rare, systemic bacterial infection that can cause arthritis, conjunctivitis, and other systemic symptoms, including gastrointestinal and neurological manifestations. It is much less likely but should be considered in the differential due to its rarity and potential for severe consequences if missed.