A 10 month old unimmunized child develops fever and a diffuse maculopapular rash on day 4 of fever starting from the face and spreading downwards. There is postauricular lymphadenopathy. What is the most likely diagnosis?
**Core Concept**
Erythema infectiosum, also known as fifth disease, is a common viral exanthem caused by parvovirus B19. It typically affects children and is characterized by a distinctive 'slapped-cheek' appearance and a subsequent lacy maculopapular rash.
**Why the Correct Answer is Right**
The child's symptoms are classic for erythema infectiosum. The fever and diffuse maculopapular rash starting from the face and spreading downwards, along with postauricular lymphadenopathy, are consistent with parvovirus B19 infection. The virus primarily affects erythrocyte precursors, leading to mild anemia in some cases. In individuals with underlying hemoglobinopathies, it can cause a more severe aplastic crisis.
**Why Each Wrong Option is Incorrect**
* **Option A:** Measles is a viral exanthem that typically presents with high fever, cough, coryza, and a maculopapular rash that starts on the head and spreads downwards. However, postauricular lymphadenopathy is not a characteristic feature of measles.
* **Option B:** Rubella is a viral exanthem that presents with a maculopapular rash, fever, and lymphadenopathy. However, the rash typically starts on the face and spreads downwards, but the postauricular lymphadenopathy is not a prominent feature.
* **Option C:** Scarlet fever is a bacterial exanthem caused by group A beta-hemolytic streptococci. It presents with a characteristic 'strawberry tongue', fever, and a sandpapery rash. However, postauricular lymphadenopathy is not a feature of scarlet fever.
**Clinical Pearl / High-Yield Fact**
Parvovirus B19 infection can cause a more severe disease in individuals with underlying hemoglobinopathies, such as sickle cell disease or beta-thalassemia. In these cases, the infection can lead to a transient aplastic crisis, characterized by a sudden decrease in red blood cell production.
**Correct Answer: D. Erythema infectiosum.**