Complex sentence: Psoralen and ultraviolet A light (PUVA) treatment involves taking a drug that increases the skin's sensitivity to ultraviolet light, then exposed to high doses of ultraviolet A light.
Split into simpler sentences: Psoralen and ultraviolet A light (PUVA) treatment involves taking a drug which increases the skin's sensitivity to ultraviolet light. The skin is then exposed to high doses of ultraviolet A light.