**Core Concept**
Ustekinumab is a **monoclonal antibody** used in the treatment of various autoimmune diseases. It targets the **p40 subunit** of **interleukin-12 (IL-12)** and **interleukin-23 (IL-23)**, which are **cytokines** involved in the inflammatory response. This mechanism is crucial in conditions characterized by excessive inflammation.

**Why the Correct Answer is Right**
Ustekinumab's mechanism of action involves binding to the p40 subunit shared by IL-12 and IL-23, thereby inhibiting the activities of these cytokines. This inhibition is particularly beneficial in treating diseases like **psoriasis** and **Crohn's disease**, where IL-12 and IL-23 play significant roles in the pathogenesis by promoting **T-helper 1 (Th1) cell** and **T-helper 17 (Th17) cell** responses, respectively.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while ustekinumab is used for autoimmune diseases, the specific application depends on the disease's pathophysiology and the role of IL-12 and IL-23.
**Option B:** Incorrect as it does not specify a condition where ustekinumab's mechanism of action would be beneficial.
**Option C:** Incorrect because, although ustekinumab is used in dermatology, its use is more specific to conditions like psoriasis.
**Option D:** Incorrect as it is not a recognized indication for ustekinumab.

**Clinical Pearl / High-Yield Fact**
Ustekinumab is a key treatment option for **moderate to severe psoriasis** and **Crohn's disease**, offering an alternative for patients who have failed other therapies. Its use underscores the importance of understanding the **immunological basis** of diseases.

**Correct Answer:** D. Psoriasis