This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
The peptide binding site on Class I MHC molecules is located in: 
Select and explain. 
A. Proximal domain of a subunits, B. Distal domain of a subunit, C. Proximal domains of a and p subunit, D. Distal domains of a and p subunit
B: Distal domain of a subunit
Answer: B: Distal domain of a subunit 
Explanation: Major histocompatibility complex (MHC) molecules are found in one region of chromosome 6. They are divided into class I and class II. Peptide binding site on class I MHC molecules is located in a grove between the distal domains of a subunits (between a1& a 2 domains). CD8 Cytotoxic T lymphocytes can recognize peptide antigen only when presented along MHC I molecules. Ref: Interface between Chemistry and Biochemistry By Pierre Jolles, Page 106; Textbook of Microbiology By Ananthanarayan and Panicker, 7th Edition, Page 130.