**Core Concept**
PUVA therapy is a form of phototherapy that combines psoralen (a light-sensitive medication) with ultraviolet A (UVA) radiation to treat skin disorders. It works by increasing the sensitivity of the skin to UVA light, which helps to reduce inflammation and slow down skin cell growth.

**Why the Correct Answer is Right**
PUVA therapy is commonly used to treat conditions such as psoriasis, vitiligo, and mycosis fungoides, a type of cutaneous T-cell lymphoma. The psoralen increases the sensitivity of the skin to UVA light, which then activates the skin cells and reduces inflammation. This treatment is particularly effective for skin conditions that are resistant to other forms of therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PUVA therapy is indeed used to treat psoriasis, which is a common skin condition characterized by thick, scaly patches on the skin.
**Option B:** This option is incorrect because vitiligo is a condition in which the skin loses its pigment, and PUVA therapy is actually used to treat this condition by increasing the sensitivity of the skin to UVA light.
**Option C:** This option is incorrect because mycosis fungoides is a type of cutaneous T-cell lymphoma that can be treated with PUVA therapy to slow down the growth of skin cells and reduce inflammation.

**Clinical Pearl / High-Yield Fact**
When using PUVA therapy, it is essential to use protective eyewear to prevent damage to the eyes, as UVA radiation can cause cataracts and other eye problems.

**Correct Answer: D.**