**Core Concept**
Acanthosis refers to the thickening of the stratum spinosum layer of the epidermis, a characteristic histopathological feature of certain skin conditions. This process involves an increase in the number of keratinocytes, leading to a thickened epidermis.

**Why the Correct Answer is Right**
Acanthosis is a hallmark feature of various dermatological conditions, including psoriasis, lichen planus, and keratosis pilaris. The thickening of the stratum spinosum layer is due to an increase in the number of keratinocytes, which is mediated by the activation of keratinocyte proliferation pathways. The stratum spinosum layer plays a crucial role in the formation of the epidermal barrier, and its thickening can lead to a range of clinical manifestations, including scaling, thickening, and roughening of the skin.

**Why Each Wrong Option is Incorrect**
**Option A:** Acanthosis is not associated with the thickening of the dermal layer, but rather the epidermal layer.
**Option B:** Hyperpigmentation is a feature of certain skin conditions, but it is not directly related to acanthosis.
**Option C:** Acanthosis is not a feature of atopic dermatitis, which is characterized by a different set of histopathological features.

**Clinical Pearl / High-Yield Fact**
Acanthosis is a key histopathological feature of psoriasis, a chronic skin condition that affects millions of people worldwide. The thickening of the stratum spinosum layer is a critical diagnostic criterion for psoriasis, and it can be distinguished from other skin conditions using histopathological examination.

**Correct Answer:** D.