Gastric proteolysis requires which of the following substances?

Gastric proteolysis, which is the process of breaking down proteins into smaller peptides and amino acids within the stomach, primarily relies on hydrochloric acid. Hydrochloric acid plays a critical role in creating an acidic environment in the stomach, which is necessary for the activation of pepsinogen to pepsin, the enzyme that digests proteins. The acidic conditions also help denature proteins, making them easier for enzymes to act upon.

In contrast, other options listed do not play a direct role in the process of gastric proteolysis. Pyridoxine (vitamin B6) is involved in amino acid metabolism and neurotransmitter synthesis but does not directly influence protein digestion in the stomach. Ascorbic acid (vitamin C) is primarily known for its antioxidant properties and is involved in collagen synthesis but is not required for gastric proteolysis. The intrinsic factor, on the other hand, is crucial for the absorption of vitamin B12 in the intestine but does not contribute to the digestion of proteins in the stomach. Therefore, hydrochloric acid is the essential substance needed for effective gastric proteolysis.