**Core Concept**
Psoriasis is a chronic inflammatory skin condition characterized by abnormal immune response and keratinocyte proliferation. The pathogenesis involves an interplay of genetic, environmental, and immune factors, leading to the formation of erythematous, scaly plaques. The condition affects various skin areas, with predilection for certain sites.

**Why the Correct Answer is Right**
The most common site affected by psoriasis is the extensor surfaces of the elbows and knees, particularly the knee joint. This is due to the accumulation of inflammatory cells, such as T lymphocytes and dendritic cells, in these areas, leading to keratinocyte proliferation and epidermal thickening. The involvement of the knee joint is thought to be related to the presence of joint effusions and synovial inflammation, contributing to the severity of the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Scalp involvement is common in psoriasis, but it is not the most common site affected.
**Option B:** Pityriasis rosea, a different skin condition, often presents with a herald patch on the trunk, not the most common site for psoriasis.
**Option C:** Psoriatic nail changes are a common feature of the disease, but they are not the most common site affected.

**Clinical Pearl / High-Yield Fact**
The "daisy sign" is a characteristic feature of psoriatic arthritis, where the proximal interphalangeal joints are involved in a symmetrical manner, resembling the petals of a daisy.

**Correct Answer: C. Elbows and knees.**