## **Core Concept**
The question tests knowledge of phototherapy, specifically the application of ultraviolet (UV) light in treating skin diseases. UV light is categorized into three main types: UVA, UVB, and UVC, each with different wavelengths and effects on the skin. The therapeutic use of UV light depends on its wavelength, which affects its penetration into the skin and its biological effects.

## **Why the Correct Answer is Right**
The correct answer, **PUVA (Psoralen + UVA)**, is a well-established treatment for certain skin diseases, notably psoriasis, vitiligo, and cutaneous T-cell lymphoma. UVA light has a longer wavelength (320-400 nm), allowing it to penetrate deeper into the skin. When used in conjunction with psoralen, a photosensitizing agent, PUVA therapy becomes particularly effective. Psoralen intercalates into DNA and, upon activation by UVA light, inhibits DNA synthesis, thereby reducing the proliferation of skin cells characteristic of conditions like psoriasis.

## **Why Each Wrong Option is Incorrect**
- **Option A: UVB phototherapy** is indeed used for skin diseases, particularly for psoriasis and eczema. However, it has a shorter wavelength (290-320 nm) and is more superficial in its effects compared to UVA. While effective, it's not the best range when considering the combination therapy for a broader range of skin diseases.
- **Option C: UVC** is not typically used for therapeutic purposes due to its short wavelength (less than 290 nm) and because it's largely absorbed by the Earth's atmosphere. Its use is limited and not standard for treating skin diseases.
- **Option D: Narrowband UVB (311 nm)** is a specific form of UVB phototherapy that has become popular for treating psoriasis and other skin conditions. It has a narrower spectrum than broadband UVB and is considered to have a better side effect profile. However, the question seems to focus on the combination therapy (like PUVA) which is very effective.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that PUVA therapy requires careful patient selection and monitoring due to potential side effects, including increased risk of skin cancer. It's particularly useful for patients with widespread skin involvement or those who have not responded to other treatments.

## **Correct Answer: B. PUVA.**