Differential Diagnosis for a 12-year-old Male with Recurring Joint Swelling and Redness

• Single most likely diagnosis:
  • Septic Arthritis: Initially, the symptoms improved with Keflex (an antibiotic), suggesting a bacterial infection. However, the recurrence in different joints and lack of response to antibiotics later on complicates this diagnosis. The initial response to Keflex could have been due to a bacterial cause, but the subsequent lack of response and the pattern of recurrence suggest other possibilities.
• Other Likely diagnoses:
  • Juvenile Idiopathic Arthritis (JIA): The recurring nature of the symptoms in different joints, the presence of swelling and redness, and the initial mild discomfort are consistent with JIA. The lack of systemic symptoms (like fever) and the failure of topical steroids could also fit within the spectrum of JIA, particularly oligoarticular JIA.
  • Psoriatic Arthritis: Although less common, the scabbing of the skin as the swelling decreased could hint at psoriatic changes, and psoriatic arthritis can present with oligoarticular involvement. However, the absence of clear psoriatic skin lesions or a family history makes this less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Lyme Arthritis: Given the potential for joint involvement and the possibility of initial improvement with antibiotics (if the patient had been misdiagnosed with a bacterial infection), Lyme disease should be considered, especially if the patient lives in an endemic area. The lack of response to antibiotics later on does not rule out Lyme, as it can have a variable response to treatment.
  • Gonococcal Arthritis: Although less likely in a 12-year-old without a clear history of sexual activity, it's crucial to consider in any case of septic arthritis, especially with the initial response to antibiotics. However, the subsequent lack of response and the clinical context make this less probable.
• Rare diagnoses:
  • Sarcoidosis: This can cause arthritis in children, although it's rare. The lack of systemic symptoms and other organ involvement makes this less likely.
  • Reactive Arthritis: Typically follows a gastrointestinal or genitourinary infection, which was not mentioned in the history. The pattern of joint involvement and the absence of other symptoms (like conjunctivitis or urethritis) make this less likely.

Each diagnosis is considered based on the pattern of joint involvement, response to treatment, and the presence or absence of systemic symptoms. The key to narrowing down the differential lies in further history (like exposure to ticks for Lyme disease), physical examination findings (such as psoriatic skin lesions), and potentially, diagnostic tests (like blood work for inflammatory markers, Lyme titers, or joint fluid analysis).