Differential Diagnosis for a 4-year-old Girl with Left Knee Pain and Systemic Symptoms

• Single most likely diagnosis
  • Serum sickness-like illness: This condition is characterized by a morbilliform rash, urticaria, and arthritis, often occurring after the use of certain medications, including antibiotics. The timing of the symptoms in relation to the antibiotic use and the combination of rash, joint swelling, and fever make this diagnosis the most likely.
• Other Likely diagnoses
  • Immunoglobulin A vasculitis: This condition, formerly known as Henoch-Schönlein purpura, can present with rash, joint pain, and fever, but it typically includes gastrointestinal symptoms and a more distinctive purpuric rash, which is not mentioned in the scenario.
  • Postviral arthritis: While possible, the direct temporal relationship with antibiotic use and the specific constellation of symptoms (including a widespread morbilliform rash and urticaria) make this less likely than serum sickness-like illness.
• Do Not Miss diagnoses
  • Systemic juvenile idiopathic arthritis (sJIA): Although less likely given the context of recent antibiotic use, sJIA can present with fever, rash, and joint inflammation. Missing this diagnosis could lead to significant morbidity due to delayed treatment.
  • Stevens-Johnson syndrome: This is a severe condition characterized by skin and mucous membrane lesions, often following medication use or infections. While the patient has a rash and fever, the absence of mucous membrane involvement and the nature of the rash (morbilliform rather than the typical target lesions) make this less likely. However, it's crucial not to miss this diagnosis due to its potential severity and the need for immediate withdrawal of the offending agent and supportive care.
• Rare diagnoses
  • Other autoimmune or rheumatologic conditions could present with similar symptoms but are less common in this age group and with this specific set of symptoms, making them less likely without additional specific findings.