I'm not sure if the answer is right. Could be:
"If I made a sandwich with lettuce and cheese and tomato and egg, they would exist!"
Do sandwiches with lettuce and cheese and tomato and egg exist?
If I made a sandwich with lettuce and cheese and tomato and egg, it would exist!
"It would exist" is correct because the ingredients becomes a single sandwich.
I read it out differently because when you list items you only say "and" before the very nice item.
"I just bought some eggs, carrots, sugar, broccoli and chocolate."