Differential Diagnosis for Morbilliform Rash on the Back
- Single most likely diagnosis
- Viral Exanthem (e.g., measles, rubella, roseola): This is a common cause of morbilliform rash, especially in children and young adults, often accompanied by fever and other systemic symptoms.
- Other Likely diagnoses
- Drug Reaction: Many medications can cause morbilliform drug eruptions, which are among the most common types of drug rashes. These can occur at any age and are often related to the use of new medications.
- Allergic Contact Dermatitis: While typically more localized, in severe cases, allergic contact dermatitis can spread and present with a morbilliform appearance, especially if the allergen is widespread (e.g., nickel, certain plants).
- Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition can sometimes present with a morbilliform rash, especially if the patient is taking ampicillin or other penicillins.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
- Meningococcemia: Although rare, this condition can present with a rash that may initially appear morbilliform before progressing to the more characteristic petechial or purpuric lesions. It is life-threatening and requires immediate medical attention.
- Secondary Syphilis: The rash of secondary syphilis can be morbilliform and is often seen on the trunk, including the back. It's crucial to consider this diagnosis due to the potential for serious complications if left untreated.
- Kawasaki Disease: While more commonly associated with children, Kawasaki disease can cause a morbilliform rash among other symptoms like fever, lymphadenopathy, and mucous membrane changes. It's a do-not-miss diagnosis due to the risk of cardiac complications.
- Rare diagnoses
- Erythema Infectiosum (Fifth Disease): Caused by parvovirus B19, this condition typically presents with a "slapped-cheek" appearance but can also cause a morbilliform rash on the body, including the back.
- Pityriasis Rosea: Although the rash is typically more localized and has a characteristic "herald patch," in some cases, it can spread and appear morbilliform, especially on the trunk.