**Core Concept:**

The process of digestion involves a series of complex enzymatic and mechanical actions that break down complex molecules into simpler forms that can be absorbed and utilized by the body. In the context of this question, we are focusing on the initiation of protein digestion. Proteins are large biomolecules composed of amino acids, and their digestion involves specific enzymes that act upon them.

**Why the Correct Answer is Right:**

Protein digestion begins in the stomach, where gastric acid and the enzyme pepsin are involved. Pepsin is a protease enzyme that is activated in the presence of low pH, as found in the stomach. It breaks down proteins into smaller peptides and amino acids. The correct answer, D (**Digestion of proteins is initiated by gastric acid and pepsin**), highlights this crucial aspect of protein digestion, emphasizing the role of gastric acid and pepsin in the initiation of protein breakdown.

**Why Each Wrong Option is Incorrect:**

A) **Gut enzymes:** While these enzymes are essential for further digestion, they are not involved in the initiation of protein digestion. Gut enzymes act primarily on carbohydrates, fats, and peptides, but not proteins.

B) **Salivary enzymes:** Although salivary amylase is involved in carbohydrate digestion, it does not initiate protein digestion. Protein digestion begins in the stomach.

C) **Liver enzymes:** The liver produces enzymes like alkaline phosphatase and bilirubin, which are involved in detoxification and bile production, respectively. These enzymes are not responsible for initiating protein digestion.

**Clinical Pearl:**

Understanding the correct pathway of digestion is essential for medical students and healthcare professionals alike. The process involves a series of organs and enzymes working together to break down complex molecules into simpler forms for absorption and utilization by the body. For instance, the pancreas secretes enzymes like trypsin and chymotrypsin for the further digestion of proteins in the small intestine, but the correct initiation of protein digestion occurs in the stomach.