Q. A professor describes 'ketogenic amino acids' to her students. She explains to her intrigued medical students that these unique amino acids can be metabolized into ketone bodies or fatty acids but cannot be converted into glucose through gluconeogenesis. This is contrasted with 'glucogenic amino acids,' which can be metabolized into glucose. The professor then throws a challenge to her students, asking, "Which one of the following amino acids is classified as purely ketogenic?"