What spirit is mixed with ginger beer in a Moscow mule

Answer
Vodka

A Moscow mule, also known as a Vodka buck, is a buck made with a high quality vodka, a spicy ginger beer, and lime juice, garnished with a slice or wedge of lime.

It is usually served in a copper mug. The cocktail became popular during the vodka craze in the United States during the 1950s.

The name refers to the popular perception of vodka as a Russian product.

Read more...https://en.wikipedia.org/wiki/Moscow_mule