Differential Diagnosis for a 14-month-old with Fever and Red Warm Rash to Left Knee

• Single most likely diagnosis:
  • Viral infection (such as roseola or a viral exanthem): This is a common cause of fever and rash in infants, and the localized nature of the rash could be an initial presentation before it becomes more widespread.
• Other Likely diagnoses:
  • Bacterial cellulitis: Given the localized redness and warmth, especially if there's a history of trauma or insect bite, bacterial cellulitis is a plausible diagnosis.
  • Erythema infectiosum (Fifth disease): Although it typically presents with a "slapped cheek" appearance, it can start with a localized rash and is accompanied by fever.
  • Allergic contact dermatitis: If the child has been exposed to a new substance or plant, an allergic reaction could cause a localized rash.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed):
  • Osteomyelitis or septic arthritis: These conditions are medical emergencies that require prompt diagnosis and treatment. The presence of fever and a localized, warm rash near a joint could indicate an infection within the bone or joint.
  • Necrotizing fasciitis: Although rare, this condition is life-threatening and can present with a painful, red, warm rash, often accompanied by fever.
• Rare diagnoses:
  • Kawasaki disease: While it typically presents with more systemic symptoms and rash, it's a rare condition that can cause fever and rash and is important to consider, especially if other symptoms like conjunctivitis or changes in the lips and mouth are present.
  • Juvenile idiopathic arthritis (JIA): Certain types of JIA can present with fever and rash, though this would be less common and typically part of a more complex clinical picture.