How does pepsinogen change into its active form?
(a) In the presence of hydrochloric acid
(b) In the presence of trypsin
(c) In the presence of bile
(d) In the presence of carboxypeptidase

Hint: In the stomach, the inactive proenzyme pepsinogen gets converted into active enzyme pepsin which converts protein into amino acids. Pepsinogen is a proenzyme secreted by chief cells.

Complete answer:
Pepsinogen is a digestive enzyme that helps within the digestion of proteins. The hydrochloric acid secreted in the stomach provides the optimum pH, which activates the pepsinogen enzyme required to digest proteins. Enzyme pepsinogen is converted to pepsin, which converts the proteins into amino acids.

Additional information:
Pepsin is an endopeptidase, which fragments down proteins into smaller peptides. It is produced within the chief cells of the stomach lining and is one among the most digestive enzymes within the digestive systems of humans, where it helps digest the proteins in food.
It is one of three principal proteases within the human digestive system, the other two being chymotrypsin and trypsin. During the method of digestion, these enzymes, each of which is specialized in cleaving links between particular sorts of amino acids. They cooperate to break down dietary proteins into their components, i.e., peptides and amino acids, which can be readily absorbed by the small intestine. The cleavage particularly of pepsin is wide, but some amino acids like tyrosine, phenylalanine, and tryptophan increase the probability of cleavage.

So, the correct answer is ‘In the presence of hydrochloric acid’.

Note:
It was the first animal enzyme to be discovered. Theodor Schwann discovered it in 1836. Pepsin is highly active in acidic environments between pH 1.5 to 2.5. It is inactive at pH 6.5 and above, however pepsin isn't fully denatured or irreversibly inactivated until pH 8.0. Renin is another enzyme, which is present in the gastric juice of infants and this helps in the digestion of milk protein.