**Core Concept**
Psoriasis and psoriatic arthritis are chronic inflammatory autoimmune diseases characterized by the involvement of various immune cells and cytokines. Biologic agents targeting tumor necrosis factor-alpha (TNF-α) have been widely used to treat these conditions by modulating the immune response.

**Why the Correct Answer is Right**
TNF-α inhibitors work by binding to TNF-α, preventing its interaction with cell surface receptors, and thereby reducing inflammation and immune cell activation. Examples of anti-TNF-α biologic agents used in psoriasis and psoriatic arthritis include etanercept, adalimumab, and infliximab. These agents have been shown to improve symptoms and quality of life in patients with these conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Etanercept is an anti-TNF-α biologic agent used to treat psoriasis and psoriatic arthritis. It is a fusion protein that binds to TNF-α, preventing its interaction with cell surface receptors.
**Option B:** Adalimumab is also an anti-TNF-α biologic agent used to treat psoriasis and psoriatic arthritis. It is a fully human monoclonal antibody that targets TNF-α.
**Option C:** Ustekinumab is a biologic agent used to treat psoriasis and psoriatic arthritis, but it targets IL-12 and IL-23, not TNF-α. It works by binding to these cytokines, preventing their interaction with cell surface receptors, and thereby reducing inflammation and immune cell activation.

**Clinical Pearl / High-Yield Fact**
Ustekinumab is a useful alternative to anti-TNF-α agents for patients with psoriasis or psoriatic arthritis who have failed or are intolerant to TNF-α inhibitors.

**Correct Answer:** C. Ustekinumab