**Question:** Least common site involvement in psoriasis is

A. Eyelids
B. Genitals
C. Nails
D. Mucosal surfaces

**Core Concept:** Psoriasis is a chronic inflammatory skin disease characterized by excessive cell division and impaired apoptosis of keratinocytes, leading to the development of scaly, red plaques. The disease primarily affects skin and nails but can also involve other sites like mucosal surfaces, hair follicles, and joints.

**Why the Correct Answer is Right:** Among the options provided, the least common site involvement in psoriasis is the mucosal surfaces (D). Mucosal surfaces are the moist, inner surfaces of the body, such as the mouth, nose, and genitalia. While psoriasis can affect these areas, they are less frequently involved compared to skin and nails.

**Why Each Wrong Option is Incorrect:**

A. Eyelids (Eyelid psoriasis, also called Psoriasis Vulgaris, is relatively common and can be easily mistaken for other eyelid disorders. This option is incorrect because eyelids are a common site for psoriasis involvement.)

B. Genitals (Psoriasis can affect genital regions, but it is uncommon compared to skin involvement. This option is incorrect because genital involvement is relatively less frequent than other sites.)

C. Nails (Psoriasis can affect nails, with nail psoriasis being a common complication of cutaneous psoriasis. This option is incorrect because nail involvement is more common than mucosal surfaces.)

**Clinical Pearl:** While psoriasis can affect various body sites, some areas are more commonly involved than others. Understanding these patterns can help in recognizing psoriasis and distinguishing it from other skin conditions.

**Correct Answer:** D. Mucosal surfaces (D)

**Explanation:** Psoriasis can affect mucosal surfaces, but they are less commonly involved compared to skin and nails. This is why this option is correct among the given choices. Understanding the varying degrees of involvement in psoriasis can aid in recognizing and accurately diagnosing this condition, particularly in cases where skin involvement is minimal or absent.