In human gastrointestinal physiology, where does chemical protein digestion begin (i.e., where are proteins first enzymatically cleaved into peptides)?

• A. Mouth, primarily via salivary amylase
• B. Large intestine, primarily via pancreatic trypsin
• C. Stomach, primarily via pepsin activated in acidic gastric juice
• D. Small intestine lumen (duodenum), primarily via brush-border disaccharidases

Answer: (C)