## **Core Concept**
Mendelian inheritance refers to the pattern of inheritance seen for traits determined by a single gene with two alleles, one inherited from each parent. These diseases follow the classic patterns described by Gregor Mendel, including autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive inheritance.

## **Why the Correct Answer is Right**
The most common group of diseases following Mendelian inheritance are **Autosomal Dominant (AD) disorders**. This is because a single copy of the dominant allele is enough to cause the condition, making it more likely for the trait to be expressed in offspring. AD disorders include conditions like Huntington's disease, Marfan syndrome, and neurofibromatosis type 1. These conditions are relatively common in the population and have a high penetrance, meaning that if you have the mutation, you are likely to express the condition.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Autosomal Recessive (AR) disorders** are significant and include conditions like cystic fibrosis and sickle cell disease, they are less common than AD disorders in the population because they require an individual to be homozygous recessive to express the disease.
- **Option C:** **X-linked Recessive disorders** are less common than AD disorders and primarily affect males (who have one X chromosome), as females would need to be homozygous for the mutation to express the condition. Examples include hemophilia A and Duchenne muscular dystrophy.
- **Option D:** Although **Multifactorial/Polygenic disorders** are common, they do not follow a simple Mendelian pattern of inheritance. Instead, they are influenced by multiple genes and environmental factors.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Autosomal Dominant disorders** often have a family history, and a parent has a 50% chance of passing the mutated gene to each offspring. This pattern can help in identifying and counseling families about the risk of these conditions.

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