**Core Concept**
Psoriasis is a chronic inflammatory skin condition characterized by accelerated epidermal turnover and abnormal immune response. It is a complex disease involving the interplay of keratinocytes, T lymphocytes, and various cytokines.

**Why the Correct Answer is Right**
The hallmark of psoriasis is the premature differentiation of keratinocytes, leading to thickening of the epidermis. This is mediated by the abnormal expression of various genes, including those involved in cell cycle regulation and apoptosis. The disease is also characterized by an immune response, with the activation of T lymphocytes, particularly CD4+ T cells, which release cytokines such as TNF-alpha and IL-17, promoting inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because while erythema and scaling are common features of psoriasis, they are not the most important or defining characteristics of the disease.
**Option B:** This is incorrect because although psoriasis can involve the joints, it is primarily a skin condition and not a rheumatological disease.
**Option C:** This is incorrect because while psoriasis can be associated with various systemic comorbidities, such as cardiovascular disease and diabetes, these are not the defining features of the disease.

**Clinical Pearl / High-Yield Fact**
Psoriasis is a heterogeneous disease with multiple subtypes, including plaque, guttate, inverse, and pustular psoriasis. Recognizing the different clinical presentations is essential for accurate diagnosis and management.

**Correct Answer: D. Accelerated epidermal turnover.**