## Core Concept
6-mercaptopurine (6-MP) is an **antimetabolite** used primarily in the treatment of **acute lymphoblastic leukemia (ALL)** and **inflammatory bowel disease (IBD)**. It acts by inhibiting **purine synthesis**, which is crucial for DNA and RNA production in rapidly dividing cells.

## Why the Correct Answer is Right
The correct answer is related to the mechanism or a characteristic of 6-mercaptopurine that does not align with the properties of the drug. 6-MP is metabolized by **xanthine oxidase** to an inactive metabolite. It also requires **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)** for its activation. One of its significant side effects is **myelosuppression**, and it can cause **pancreatitis** and **hepatotoxicity**.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, we cannot directly assess its accuracy in relation to 6-MP. However, if it aligns with known properties or effects of 6-MP, it would be incorrect as an "EXCEPT" question.
- **Option B:** Similarly, without specifics, if B is a true statement about 6-MP, it would be incorrect in the context of the question.
- **Option C:** If option C is a recognized effect or characteristic of 6-MP, then it would not be the correct answer to the "EXCEPT" question.
- **Option D:** Given as the correct answer, implying that the statement about 6-MP in option D is not true.

## Clinical Pearl / High-Yield Fact
A critical point to remember is that **allopurinol**, a xanthine oxidase inhibitor used to treat gout, can **increase the levels of 6-mercaptopurine** by reducing its metabolism, thereby increasing the risk of toxicity. This interaction is crucial for clinicians to manage patients on both medications effectively.

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