**Core Concept**
Atopic dermatitis, also known as eczema, is a chronic inflammatory skin condition characterized by dry, itchy, and scaly skin lesions. It typically presents in childhood and is associated with a strong family history of atopy, including asthma and allergies.

**Why the Correct Answer is Right**
The rash in atopic dermatitis typically appears on the extensor surfaces of the elbows, knees, and ankles in infants and young children. This is due to the presence of allergens and irritants that trigger an immune response and lead to inflammation. The involvement of the extensor surfaces is a key diagnostic feature of atopic dermatitis, and it is often accompanied by signs of dry skin and increased sensitivity to environmental allergens.

**Why Each Wrong Option is Incorrect**
**Option A:** Erythema infectiosum is a viral illness characterized by a distinctive "slapped cheek" rash, which typically appears on the cheeks and not the extensor surfaces.
**Option B:** Impetigo is a bacterial skin infection that presents with honey-colored crusting and usually affects the face and extremities but not typically the extensor surfaces.
**Option C:** Contact dermatitis is an allergic reaction to an external agent, which can cause a rash on any area of the body but is not typically characterized by the specific pattern of extensor surface involvement seen in atopic dermatitis.

**Clinical Pearl / High-Yield Fact**
Atopic dermatitis is a key component of atopic march, a sequence of allergic diseases that begins in infancy and progresses to asthma and allergic rhinitis in later childhood.

**Correct Answer:** D.