One of the constituents of the pancreatic juice while poured into the duodenum in humans is

The correct option is trypsinogen.

To understand why this is the correct answer, let's break down the physiology of pancreatic secretion and activation step-by-step:

1. Production and Secretion of Pancreatic Enzymes:
The pancreas is a vital exocrine gland that secretes pancreatic juice containing digestive enzymes. To prevent the pancreas from digesting itself (autodigestion), its proteolytic (protein-digesting) enzymes are synthesized and stored in an inactive form called zymogens or proenzymes. Among these inactive precursors is trypsinogen, along with chymotrypsinogen and procarboxypeptidase.

2. Release into the Duodenum:
When pancreatic juice is poured into the duodenum (the first part of the small intestine), the enzymes are still in their inactive states. Thus, trypsinogen is poured directly into the duodenum in its inactive form.

3. Activation in the Duodenum:
Once inside the duodenum, trypsinogen comes into contact with an enzyme called enterokinase (also known as enteropeptidase), which is secreted by the intestinal mucosa. Enterokinase cleaves a specific peptide bond in trypsinogen to convert it into its active form, trypsin. Active trypsin then goes on to activate other zymogens like chymotrypsinogen into chymotrypsin.

4. Evaluating the Options:
- trypsin: This is the active form of the enzyme. It is not poured into the duodenum in this form; it is activated inside the duodenum.
- enterokinase: This enzyme is secreted by the intestinal brush border (duodenal mucosa), not by the pancreas.
- chymotrypsin: This is also an active form, activated from chymotrypsinogen by trypsin inside the intestine.
- trypsinogen: This is the inactive constituent of pancreatic juice that is directly poured into the duodenum, making it the correct answer.