## **Core Concept**
Erythema infectiosum, also known as fifth disease, is a common viral infection primarily affecting children. It is characterized by a distinctive rash and is caused by parvovirus B19. The infection typically presents with a specific sequence of symptoms.

## **Why the Correct Answer is Right**
The correct answer involves understanding the key features of erythema infectiosum:
- **Slapped cheek appearance**: This is a hallmark of erythema infectiosum, where the rash starts on the cheeks, giving them a "slapped" appearance.
- **Caused by parvovirus**: The infection is indeed caused by parvovirus B19, which targets erythrocyte precursors in the bone marrow.
- **Defervescence before rash**: Typically, there is a prodromal phase with fever, which resolves (defervescence) before the onset of the rash.
- **Rash appears on head & neck**: While the rash can spread to other parts of the body, it characteristically starts on the cheeks.

## **Why Each Wrong Option is Incorrect**
- **Option D:** While the rash can eventually spread to other areas, stating it "appears" on the head and neck is misleading, as the characteristic initial presentation is on the cheeks.
- Without specific details on the provided options A, B, and D, we focus on the accuracy of statements a, b, and c.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that individuals with erythema infectiosum are usually contagious before the rash appears and are no longer contagious once the rash has developed. This is crucial for understanding infection control measures.

## **Correct Answer: B.**