What additional flavor is typically added to a Cosmopolitan aside from vodka and Cointreau?

In a Cosmopolitan, the additional flavor that is typically added alongside vodka and Cointreau is cranberry juice. This particular ingredient contributes a tartness and sweetness that balances the drink, complementing the citrus notes from the Cointreau and the smoothness of the vodka. The vibrant red color from the cranberry juice also enhances the visual appeal of the cocktail, making it a popular choice in bars and restaurants.

The other options do not traditionally belong in a Cosmopolitan. Orange juice, while it provides citrus flavor, is not used in this cocktail. Sprite might add sweetness and fizz, which is not characteristic of a Cosmopolitan's profile. Pineapple juice would significantly change the flavor and is not a typical ingredient in a Cosmopolitan. Thus, cranberry juice is the essential flavor that completes this classic cocktail.