**Core Concept**
Atopic dermatitis, also known as eczema, is a chronic inflammatory skin condition characterized by dry, itchy, and scaly skin. It is a type of atopic disease, which also includes asthma and allergic rhinitis. The exact pathogenesis of atopic dermatitis is complex and involves a combination of genetic, environmental, and immune system factors.

**Why the Correct Answer is Right**
The commonest site of atopic dermatitis is the face, particularly the cheeks, in infants and young children. This is due to the thin skin on the face, which is more prone to water loss and irritation. The facial skin's natural barrier function is disrupted in atopic dermatitis, leading to increased permeability and water loss. This results in dryness, itchiness, and inflammation. The correct answer is the **face**, specifically the cheeks.

**Why Each Wrong Option is Incorrect**
**Option A:** Scalp - While the scalp can be affected in atopic dermatitis, it is not the most common site, especially in infants and young children.

**Option B:** Trunk - The trunk can be involved in atopic dermatitis, but it is less common than the face, particularly in infants.

**Option C:** Extremities - The extremities, such as the arms and legs, can be affected in atopic dermatitis, but they are not the most common site.

**Clinical Pearl / High-Yield Fact**
Remember that atopic dermatitis can affect any age group, but it is most common in infants and young children, and the face is the most common site of involvement.

**Correct Answer:** C.