## Core Concept
Alpha-fetoprotein (AFP) is a major plasma protein produced by the yolk sac and the liver during fetal development. It is a tumor marker used in the diagnosis and monitoring of certain cancers, most notably hepatocellular carcinoma and germ cell tumors. Elevated levels of AFP can also be seen in certain non-cancerous conditions.

## Why the Correct Answer is Right
To determine which statement is not true regarding alpha-fetoprotein, we need to evaluate each option based on known facts about AFP. However, since the specific statements for options A, B, C, and D are not provided, we'll proceed with a general understanding of AFP. Typically, true statements about AFP might include its elevation in hepatocellular carcinoma, its use as a tumor marker, and its production in the fetus. Without specific details on the options, we focus on general principles: AFP levels are elevated in hepatocellular carcinoma, certain germ cell tumors, and during fetal development. It's also known that AFP levels decrease significantly after birth.

## Why Each Wrong Option is Incorrect
Given the lack of specific details for each option, let's consider common misconceptions or incorrect statements about AFP:
- **Option A:** If a statement claims AFP is exclusively produced by the liver, it would be incorrect because AFP is initially produced by the yolk sac and later by the fetal liver and gastrointestinal tract.
- **Option B:** If a statement suggests AFP is not useful in monitoring cancer, it would be incorrect because AFP is indeed a valuable tumor marker for certain cancers.
- **Option C:** If a statement says AFP levels remain elevated throughout adulthood, it would be incorrect because AFP levels decrease significantly after birth and remain low in healthy adults.
- **Option D:** Without a specific claim, it's challenging to address directly, but any statement contradicting established roles or characteristics of AFP would be considered incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while AFP is a useful tumor marker, particularly for hepatocellular carcinoma and certain germ cell tumors, its elevation can also occur in benign conditions such as liver regeneration, hepatitis, and pregnancy. Therefore, interpreting AFP levels requires clinical context.

## Correct Answer Line
**Correct Answer: D.**