**Core Concept**
Psoriasis is a chronic autoimmune skin condition characterized by thickened, scaly skin lesions due to abnormal keratinocyte proliferation. Certain medications can trigger or worsen psoriasis by influencing the immune system and keratinocyte growth.

**Why the Correct Answer is Right**
Beta-blockers, such as propranolol, can exacerbate psoriasis by suppressing the immune system and reducing the clearance of inflammatory mediators. This can lead to an increase in keratinocyte proliferation and exacerbate the condition. Additionally, beta-blockers can also cause vasoconstriction, which may worsen the appearance of psoriasis lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Lithium is actually used to treat psoriasis in some cases, as it has been shown to reduce the severity of the condition.
**Option B:** Steroids are commonly used to treat inflammatory conditions, including psoriasis, and are not known to exacerbate the condition.
**Option C:** Antihistamines are not directly associated with exacerbating psoriasis, although they may have a mild effect on the immune system.

**Clinical Pearl / High-Yield Fact**
Beta-blockers can also cause other skin conditions such as lichen planus and pemphigus, making them a high-risk medication for patients with autoimmune skin conditions.

**Correct Answer:** C. Propranolol