What is the hardest exam in Korea?

I'm curious, what exactly is considered the toughest exam in Korea? Is it a standardized test for university entrance, or perhaps a specialized licensing exam for a particular profession? How does the difficulty of this exam compare to other exams around the world? Are there any unique challenges or requirements that make it particularly difficult for students to pass? I'd love to hear more about the preparation process and the impact that passing or failing this exam can have on a person's future prospects.