What ingredient in tanning lotions helps to darken the skin?

DHA (Dihydroxyacetone) is the correct answer because it is a colorless sugar that interacts with the amino acids in the skin's surface layer, leading to the formation of a brown color as a result of this chemical reaction. This process mimics the appearance of a natural tan without the need for UV exposure.

While bronzers can provide an immediate tan effect, they do not actually change the skin's pigmentation long-term; they simply add color temporarily until washed off. Carotene, found in some tanning products, primarily contributes to skin pigmentation through natural dietary sources but is not typically used in tanning lotions for the purposes of immediate darkening. Vitamin E is often included in tanning products for its skin-nourishing and antioxidant properties but does not have a role in darkening the skin. Hence, DHA is the primary ingredient responsible for achieving a darker skin tone in tanning lotions.