**Core Concept**
Psoriasis is a chronic autoimmune skin condition characterized by thickened, scaly plaques due to accelerated epidermal turnover. The disease predominantly affects the skin, but can also involve various internal organs, making it a multi-systemic disorder.

**Why the Correct Answer is Right**
The least common site involvement in psoriasis is the oral mucosa. This is because the pathogenesis of oral psoriasis is closely related to the presence of human leukocyte antigen (HLA) class I and II molecules on the epithelial cells of the oral mucosa, which triggers an immune response leading to inflammation and tissue damage. The oral mucosa is relatively resistant to the effects of psoriasis due to its unique immunological and genetic characteristics.

**Why Each Wrong Option is Incorrect**
**Option A:** Scalp involvement is relatively common in psoriasis, particularly in the form of psoriatic plaques. While scalp involvement is more common than oral mucosa involvement, it is not the least common site.

**Option B:** Nail involvement is also relatively common in psoriasis, with up to 50% of patients experiencing some degree of nail disease. Nail psoriasis can manifest as pitting, onycholysis, or subungual hyperkeratosis.

**Option C:** Joint involvement, particularly in the form of psoriatic arthritis, is a significant complication of psoriasis. Psoriatic arthritis occurs in approximately 10-30% of patients with psoriasis, making it a relatively common site of involvement.

**Clinical Pearl / High-Yield Fact**
The presence of psoriatic arthritis is a significant predictor of joint damage and disability in patients with psoriasis. Early recognition and treatment of psoriatic arthritis are essential to prevent long-term joint damage and improve patient outcomes.

**Correct Answer:** D.