What is the primary alcoholic ingredient in a Salty Dog?

The primary alcoholic ingredient in a Salty Dog is vodka. This cocktail is typically made by combining vodka with grapefruit juice, resulting in a refreshing drink with a slightly tangy flavor. The distinctive aspect of a Salty Dog is that the rim of the glass is often coated with salt, which enhances the overall taste experience by balancing the sweetness of the grapefruit juice and the crispness of the vodka.

In the context of the other options, gin, whiskey, and Midori are not traditional ingredients for this specific cocktail. While gin could create a variation that might be referred to as a Salty Dog, it is not the standard recipe. Whiskey doesn't fit with the fruity and refreshing profile that defines a Salty Dog, and Midori, being a sweet melon liqueur, is not used in this cocktail at all. Therefore, vodka stands out as the correct choice for this drink.