Correct Option
Assertion (A) states that all proteins are digested only in the small intestine. This is incorrect. Protein digestion commences in the stomach, where the enzyme pepsin breaks down proteins into proteoses and peptones. The process continues in the small intestine, where pancreatic enzymes (like trypsin and chymotrypsin) and intestinal enzymes further digest these into amino acids. Therefore, protein digestion is not exclusively confined to the small intestine.
Reason (R) states that protein-digesting enzymes from the pancreas are released into the small intestine. This is correct. The pancreas secretes inactive proteolytic enzymes, such as trypsinogen and chymotrypsinogen, into the duodenum (part of the small intestine). These are activated there (e.g., trypsinogen by enterokinase into trypsin) and play a vital role in breaking down proteins and peptides.
Since Assertion (A) is false and Reason (R) is true, option 4 is the correct choice.
Incorrect Options
Options 1 and 2 are incorrect because they both state that Assertion (A) is true, which is factually inaccurate as protein digestion begins in the stomach.
Option 3 is incorrect because it states that Reason (R) is false, whereas the pancreatic release of protein-digesting enzymes into the small intestine is a correct physiological process.