## Core Concept
The gastric juice in a normal individual, even in a fasting state, contains various substances that help in digestion and protection of the stomach lining. Gastric juice is secreted by gastric glands in the stomach and consists of water, mucus, enzymes, and acids.

## Why the Correct Answer is Right
The correct answer, **D. Bilirubin**, is not a typical content of gastric juice. Gastric juice primarily contains:
- **Pepsinogen** (the inactive form of pepsin, a digestive enzyme)
- **Gastric amylase** (an enzyme that starts carbohydrate digestion)
- **Mucus** (which protects the stomach lining)
- **Hydrochloric acid (HCl)** (which maintains the acidic pH necessary for enzyme activation and protein denaturation)
- **Intrinsic factor** (necessary for vitamin B12 absorption in the ileum)

Bilirubin, a pigment produced during heme catabolism, is not a component of gastric juice. It is primarily found in bile, which is produced by the liver and stored in the gallbladder.

## Why Each Wrong Option is Incorrect
- **Option A:** Pepsinogen is indeed a content of gastric juice. It is secreted in its inactive form and activated to pepsin by hydrochloric acid, playing a crucial role in protein digestion.
- **Option B:** Mucus is a protective component of gastric juice, shielding the stomach lining from the acidic and enzymatic environment.
- **Option C:** Gastric amylase, though not as significant as pancreatic amylase in carbohydrate digestion, is present in gastric juice and contributes to the breakdown of carbohydrates.

## Clinical Pearl / High-Yield Fact
A key point to remember is that gastric juice's composition can change based on the state of feeding or fasting and the presence of certain conditions. However, in a normal fasting individual, the components like pepsinogen, mucus, and hydrochloric acid are consistently present.

## Correct Answer: D. Bilirubin.