What must the drink a prairie oyster contain?

A prairie oyster is a traditional beverage consisting of a egg yolk, Worcestershire sauce, vinegar and ground black pepper. The egg is broken into a glass so as not to break the yolk. The unbroken yoke causes the drink to bear a texture similar to that of an oyster. The concoction has been referred to as a traditional cure for hangovers, and has appeared in media for decades.