Which sequence correctly describes the major steps of antigen processing for MHC class I presentation?

• A. Proteasomal degradation of cytosolic proteins; TAP transport of peptides into the ER; loading onto MHC I in the ER; surface expression
• B. Endosomal processing of extracellular proteins; loading onto MHC I in the ER; TAP transport of peptides into the ER; surface expression
• C. Proteasomal degradation of cytosolic proteins; loading onto MHC II in the ER; TAP transport into the ER; surface expression
• D. Antigen processing for MHC I involves endocytosis of surface proteins and presentation on MHC II

Answer: (A)

Endogenous protein processing for MHC class I starts with proteasomal degradation of cytosolic proteins into peptides, then TAP transports those peptides into the endoplasmic reticulum, where they bind to newly synthesized MHC class I molecules and the complex is delivered to the cell surface for presentation to CD8+ T cells. This sequence matches A because it reflects the cytosolic origin of the antigen, the role of TAP in ER peptide loading, and the ER-based assembly of the peptide–MHC I complex before surface expression. The other options mix in endosomal processing, MHC II loading, or endocytosis pathways that pertain to the MHC II presentation route, not the MHC I pathway.