Within the human stomach, which digestive enzyme is chiefly responsible for breaking down proteins into smaller peptides under acidic conditions?

Introduction / Context:
Proteins in our diet must be broken down into smaller peptides and amino acids before absorption. Different enzymes act at different points along the digestive tract. This question asks which enzyme digests proteins in the stomach specifically. Recognising pepsin as the main proteolytic enzyme of the stomach is a fundamental concept in digestion and human physiology.

Given Data / Assumptions:
- The stomach has an acidic environment due to hydrochloric acid.- Several enzymes are involved in digestion: trypsin, pepsin, salivary amylase, lipase and maltase.- The question focuses on protein digestion in the stomach.- You are expected to know which enzyme is active under acidic conditions there.

Concept / Approach:
Pepsin is produced as an inactive precursor pepsinogen by the chief cells of the gastric glands. In the acidic environment of the stomach, pepsinogen is converted to active pepsin, which breaks large protein molecules into smaller peptides. Trypsin, another protein digesting enzyme, is secreted by the pancreas into the small intestine and works best in alkaline conditions. Salivary amylase acts on starch in the mouth, lipase digests fats mainly in the small intestine and maltase converts maltose into glucose there. Thus, pepsin is the key proteolytic enzyme in the stomach.

Step-by-Step Solution:
Step 1: Identify that the question specifies the stomach as the site of protein digestion.Step 2: Recall that gastric glands in the stomach secrete pepsinogen and hydrochloric acid.Step 3: Understand that pepsinogen is activated to pepsin in acid and then digests proteins into smaller peptides.Step 4: Note that trypsin, amylase, lipase and maltase work in other parts of the digestive tract rather than in the stomach.Step 5: Select option B, pepsin secreted by gastric glands in the stomach, as the correct answer.

Verification / Alternative check:
Standard diagrams of the digestive system show pepsin acting specifically in the stomach and trypsin in the small intestine.Descriptions of peptic ulcers and gastritis often mention pepsin activity in an acidic environment, reinforcing its role there.No reference places salivary amylase, pancreatic lipase or maltase as the main protein digesting enzyme in the stomach, confirming pepsin as the correct choice.

Why Other Options Are Wrong:
Option A is wrong because trypsin is a pancreatic enzyme that acts in the small intestine at alkaline pH, not in the acidic stomach.Option C is wrong because salivary amylase works on starch in the mouth and becomes inactivated by stomach acid.Option D is wrong because lipase acts mainly on fats, not proteins, and is produced by the pancreas and other glands.Option E is wrong because maltase converts maltose to glucose in the intestinal lining and has no role in initial protein digestion in the stomach.

Common Pitfalls:
Learners sometimes confuse pepsin with trypsin since both digest proteins, but they must remember that pepsin is specific to the stomach.Another pitfall is thinking that all digestion occurs only in the intestine and overlooking the important role of gastric enzymes.To keep it clear, associate the letter p in pepsin with the word peptic or stomach, and the letter t in trypsin with the intestine, which helps to separate their locations.

Final Answer:
The enzyme that digests proteins in the human stomach is pepsin, which is secreted by gastric glands and activated in the acidic gastric environment.