How does hydrochloric acid (HCl) in the stomach facilitate proteolysis by pepsin?

• A. HCl chemically cleaves polypeptide bonds directly into amino acids, and pepsin only functions to neutralize excess acid preventing over‑digestion.
• B. HCl lowers gastric pH which unfolds protein structures exposing peptide bonds and triggers conversion of pepsinogen to pepsin, the active protease that cleaves peptide bonds to generate peptides.
• C. HCl activates pancreatic enzymes that retrograde into the stomach where they hydrolyze proteins into individual amino acids without involvement of pepsin.
• D. HCl serves mainly to sterilize ingested food while pepsinogen is converted to pepsin by intestinal alkaline secretions which then digest proteins back in the duodenum.

Answer: (B)