Differential Diagnosis for Flat Rash in a 7-year-old
Single most likely diagnosis
- Viral Exanthem: This is the most likely diagnosis given the rash started on the face and spread down to the arms, legs, and torso, which is a common pattern for many viral exanthems. The lack of itchiness and pain also supports this diagnosis, as viral rashes are often non-pruritic and non-painful.
Other Likely diagnoses
- Pityriasis Rosea: Although it typically starts with a herald patch, the subsequent rash can spread to the torso, arms, and legs. It's usually non-pruritic but can sometimes cause mild itching.
- Erythema Infectiosum (Fifth Disease): Characterized by a "slapped cheek" appearance initially, followed by a maculopapular rash on the body. It's more common in children and is usually not itchy or painful.
- Kawasaki Disease: An acute febrile illness that can present with a rash, although it's more commonly associated with fever, conjunctivitis, and lymphadenopathy. The rash can vary in appearance and is not typically itchy.
Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
- Meningococcemia: A severe bacterial infection that can cause a rash, which may initially appear as non-specific macules or papules but can progress to purpura. It's critical to consider this diagnosis due to its high mortality rate if not treated promptly.
- Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Although rare, these conditions are medical emergencies. They can be triggered by infections or medications and are characterized by widespread skin necrosis and detachment, often with mucosal involvement.
Rare diagnoses
- Juvenile Pemphigus: A group of rare autoimmune skin diseases that can cause blistering rashes. While not typically the first consideration for a flat, non-itchy rash, some forms can present with non-blistering lesions.
- Lichen Planus: More commonly seen in adults, but can occur in children. It usually presents with pruritic, flat-topped, violaceous papules, which doesn't perfectly match the description given, but it's a consideration in the differential for rare skin conditions.