**Core Concept**
Psoriasis is a chronic inflammatory skin condition characterized by thickened skin plaques. Certain drugs can trigger or worsen psoriasis by influencing the immune system, inflammation, and keratinocyte proliferation.

**Why the Correct Answer is Right**
The correct answer is a drug that does not exacerbate psoriasis. The pathophysiology of psoriasis involves an abnormal immune response, leading to the activation of T-cells, the release of pro-inflammatory cytokines, and the proliferation of keratinocytes. Drugs that exacerbate psoriasis typically interfere with the immune system or promote inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This drug is a beta-blocker that can induce or exacerbate psoriasis by reducing lymphocyte function and promoting inflammation.
**Option B:** This antimalarial drug can worsen psoriasis by increasing the production of pro-inflammatory cytokines and promoting keratinocyte proliferation.
**Option C:** This drug is a lithium salt used to treat bipolar disorder, which can actually improve psoriasis symptoms by reducing the activity of T-cells and decreasing inflammation.

**Clinical Pearl / High-Yield Fact**
Psoriasis is often triggered or worsened by infections, stress, and certain medications, including beta-blockers, antimalarials, and systemic steroids.

**Correct Answer: C. Lithium**