Okay, let's tackle this question about Gardner's syndrome. The user is asking for a detailed explanation of an MCQ where the correct answer is an exception among the options. First, I need to recall what Gardner's syndrome is. It's a hereditary condition, right? Part of the familial adenomatous polyposis (FAP) spectrum. So, FAP is autosomal dominant and involves multiple adenomatous polyps in the colon, which can lead to cancer if untreated.

Wait, Gardner's syndrome isn't just about the colon. There are extracolonic manifestations. I remember that it includes osteomas, fibromas, and epidermoid cysts. Oh, and there's also an increased risk of other cancers like desmoid tumors, thyroid, and brain tumors. The gene involved is the APC gene, same as in FAP. But Gardner's is a variant of FAP, so they share the same genetic basis but with more systemic features.

Now, the question is asking which statement is NOT true. Let's think about typical features. If one of the options mentions something like "caused by a mutation in the MLH1 gene," that would be wrong because MLH1 is associated with Lynch syndrome, not Gardner's. Another possible wrong option could be absence of colonic polyps, but that's not the case because Gardner's does have them. Or maybe a statement about being a separate entity from FAP, which is incorrect because it's a variant of FAP.

So, the core concept here is that Gardner's syndrome is a variant of FAP with specific extracolonic features due to APC gene mutations. The correct answer would be the option that incorrectly attributes the cause to a different gene or denies the presence of colonic polyps. The other options would be incorrect if they mention unrelated genes or features not associated with Gardner's.

I need to structure the explanation with the required sections. Start with the core concept, explain why the correct answer is right, then go through each wrong option, and end with a clinical pearl. Make sure to use precise terms like APC gene, FAP, and specific tumors. Also, mention the autosomal dominant inheritance. The clinical pearl should highlight the key features to remember for exams, maybe a mnemonic or a common mistake.

**Core Concept** Gardner's syndrome is an autosomal dominant variant of familial adenomatous polyposis (FAP) characterized by **adenomatous colorectal polyps**, **osteomas**, **fibromas**, **epidermoid cysts**, and **desmoid tumors**. It results from **APC gene mutations**, similar to classic FAP, but includes distinctive extracolonic features.

**Why the Correct Answer is Right** The correct answer identifies a statement inconsistent with Gardner's syndrome. For example, if the option claims it is caused by **MLH1/MSH2 mutations** (associated with Lynch syndrome) or lacks colonic polyps, it is incorrect. Gardner's syndrome specifically involves **APC gene mutations** and is defined by the presence of **colorectal adenomas** alongside extracolonic manifestations like **osteomas** and **desmoid tumors**.

**Why Each Wrong Option is Incorrect**
**Option A:** *If it states "caused by APC gene mutations," this is correct*—Gardner's shares the APC mutation with FAP.

✓ Correct Answer: B. Always in 5th decade of life