**Core Concept**
The question is testing the student's knowledge of characteristic clinical presentations associated with various diseases. A "slap cheek" appearance refers to a specific type of facial erythema, which is a common clinical finding in several conditions.

**Why the Correct Answer is Right**
The correct answer is associated with Fifth disease, also known as erythema infectiosum. This condition is caused by parvovirus B19, which primarily affects children. The characteristic "slap cheek" appearance is due to the intense erythema that occurs on the cheeks, giving the face a slapped appearance. The virus infects erythrocyte precursors, leading to anemia and a transient aplastic crisis in individuals with underlying hemoglobinopathies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the characteristic facial rash of roseola infantum is typically accompanied by a high fever and occurs in infants.
**Option B:** This option is incorrect because the facial rash of measles is more diffuse and not limited to the cheeks.
**Option C:** This option is incorrect because the facial rash of scarlet fever is accompanied by a sandpapery texture and a distinctive strawberry tongue.

**Clinical Pearl / High-Yield Fact**
When assessing a child with a facial rash, it's essential to consider the patient's age, the presence of fever, and the distribution of the rash to narrow down the differential diagnosis.

**Correct Answer:** D. Fifth disease.