Differential Diagnosis for a 23-Month-Old with a Red Mark on the Back of the Leg

• Single Most Likely Diagnosis
  • Impetigo: This is a common, highly contagious skin infection that can present with erythematous vesicles and ooze serous fluid, especially in a daycare setting where the risk of transmission is higher. The absence of crusting and fever also aligns with early or mild cases.
• Other Likely Diagnoses
  • Contact Dermatitis: Given the exposure to a new environment like daycare, contact dermatitis is a possibility, especially if the child came into contact with an irritant or allergen. The presentation of erythematous vesicles and serous fluid is consistent with an allergic or irritant reaction.
  • Eczema (Atopic Dermatitis): Although typically associated with more chronic symptoms, an acute flare of eczema could present similarly, especially if the child has a history of atopic diseases.
• Do Not Miss Diagnoses
  • Child Abuse: Any unexplained skin lesion, especially in a non-ambulatory child, warrants consideration of non-accidental injury. The location and description might not immediately suggest abuse, but it's crucial to rule out given the potential severity.
  • Insect Bite or Sting: Certain insect bites or stings can cause significant local reactions, including vesiculation and serous discharge. While less common, it's essential to consider, especially if the child was outdoors.
• Rare Diagnoses
  • Bullous Pemphigoid: A rare autoimmune disease that can cause blistering skin lesions. It's less common in children but should be considered if the lesion does not respond to typical treatments.
  • Zinc Deficiency (Acrodermatitis Enteropathica): Characterized by skin lesions, including vesiculobullous eruptions, and can be associated with impaired growth and development. It's rare but important to consider in cases that do not respond to standard treatments.