Differential Diagnosis for 15-month-old with Rash

Single most likely diagnosis

• Roseola infantum: This is a common illness in infants and toddlers, characterized by a high fever followed by a rash that typically appears on the trunk, including the back, chest, and base of the neck. The age of the patient and the description of the rash make this a likely diagnosis.

Other Likely diagnoses

• Viral exanthem: Various viral infections can cause rashes in children, and the presentation can be similar to roseola. The rash's location and the preceding fever suggest a viral etiology.
• Erythema infectiosum (Fifth disease): Although more commonly associated with a "slapped cheek" appearance, Fifth disease can also present with a rash on the trunk. However, the absence of the characteristic facial rash might make this less likely.
• Measles: Now less common due to vaccination, measles starts with fever and progresses to a rash that spreads from the head down to the trunk. The vaccination status of the child would influence the likelihood of this diagnosis.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed)

• Meningococcemia: This is a severe, life-threatening infection that can cause a rash, often accompanied by fever and other systemic symptoms. The rash can appear anywhere on the body, including the trunk, and can progress rapidly.
• Staphylococcal scalded skin syndrome (SSSS): Caused by staphylococcal toxins, SSSS leads to widespread skin exfoliation and can start with a localized rash. It's more common in infants and young children.
• Kawasaki disease: An acute febrile illness that can lead to coronary artery aneurysms if not treated promptly. It's characterized by fever, rash, conjunctivitis, and changes to the lips and oral cavity.

Rare diagnoses

• Juvenile idiopathic arthritis (JIA) with systemic onset: This form of JIA can present with fever, rash, and systemic symptoms, although it's less common and typically has additional symptoms like joint pain or swelling.
• Infectious mononucleosis: Caused by Epstein-Barr virus, it can present with fever and rash, especially if the child is taking certain medications like ampicillin. However, it's less common in this age group.
• Other autoimmune or rheumatologic conditions: Conditions like systemic lupus erythematosus (SLE) are rare in young children but can present with a variety of symptoms including rash and fever.