In human body, the digestion of protein begins in which of the following organs?

The correct answer is D) Stomach. Protein digestion starts here through pepsin, a protease secreted as inactive pepsinogen by the chief cells and activated by hydrochloric acid from the parietal cells. Pepsin cleaves peptide bonds inside large protein chains, producing smaller polypeptides.

• Mouth (B): Saliva contains salivary amylase, which acts on starch, and lingual lipase, which acts on fats. It has no protein-digesting enzyme.
• Liver (A): The liver makes bile, which emulsifies fats. It secretes no enzyme that digests protein.
• Small intestine (C): This is where protein digestion is completed, by trypsin, chymotrypsin and carboxypeptidase from the pancreas, reducing peptides to amino acids and dipeptides. It is not where digestion begins.

Core concept: digestion is sequential, and each organ has its own enzymes. On the Mains side, this links to nutrition and public health, and to the effect of acid-suppressing drugs like PPIs, which can reduce the stomach's ability to start protein breakdown.

Trap: Candidates pick the small intestine because most digestion and absorption happen there. Ask where the process starts, not where it finishes.