## **Core Concept**
Gardner syndrome is a variant of Familial Adenomatous Polyposis (FAP), characterized by the presence of multiple colonic polyps, osteomas, skin and soft tissue tumors, and dental abnormalities. It is an autosomal dominant disorder. The underlying genetic defect involves mutations in the **APC gene** on chromosome 5q21-22.

## **Why the Correct Answer is Right**
The correct answer, **Autosomal dominant**, is right because Gardner syndrome exhibits a pattern of inheritance where a single copy of the mutated gene is sufficient to cause the condition. This means if one parent carries the mutated **APC gene**, each child has a 50% chance of inheriting the mutated gene and thus the syndrome. This pattern is typical for autosomal dominant disorders, which often present with a high degree of penetrance and variable expressivity.

## **Why Each Wrong Option is Incorrect**
- **Option A: Autosomal recessive** is incorrect because autosomal recessive disorders require an individual to inherit two copies of the mutated gene (one from each parent) to express the condition. Gardner syndrome does not follow this pattern.
- **Option C: X-linked dominant** is incorrect because X-linked dominant disorders are caused by mutations in genes on the X chromosome. Gardner syndrome is caused by a mutation on chromosome 5, which is not an X chromosome.
- **Option D: Mitochondrial** is incorrect because mitochondrial inheritance involves genes found in the mitochondria and typically affects the energy-requiring tissues. Gardner syndrome's genetic defect is located in a nuclear gene, not in mitochondrial DNA.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with Gardner syndrome are at high risk of developing colorectal cancer due to the presence of hundreds to thousands of adenomatous polyps in the colon. Early diagnosis and prophylactic colectomy are crucial in preventing colorectal cancer. Screening for extracolonic manifestations like osteomas and dental abnormalities can also aid in the diagnosis.

## **Correct Answer: B. Autosomal dominant**