**Core Concept**
Psoriasis is a chronic inflammatory skin condition characterized by abnormal keratinocyte proliferation and immune cell activation. It typically affects the skin, but can also involve mucocutaneous and systemic sites.

**Why the Correct Answer is Right**
The least common site involvement in psoriasis is the oral cavity. This is because the oral mucosa has a different microenvironment and immune cell composition compared to the skin, making it less susceptible to psoriasis-related inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Scalp involvement is actually very common in psoriasis, as it is one of the most frequent sites affected.
**Option B:** Nail involvement is also relatively common in psoriasis, with approximately 80% of patients experiencing some degree of nail abnormality.
**Option C:** Genital psoriasis can occur, although it is less common than skin or scalp involvement.

**Clinical Pearl / High-Yield Fact**
Psoriasis can involve any mucocutaneous site, but oral and genital involvement are less common due to differences in local immune cell composition and microenvironment.

**Correct Answer:** D. Oral cavity.